Formation of Hydrocarbon Deposits in the Northern African Basins: Geological and Geochemical Conditions

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Foreword

The North African Sahara produces hydrocarbons from two major stratigraphic intervals: the Cambro-Ordovician and the Triassic. My geological speciality is the hydrocarbon setting of the Cambro-Ordovician of North America consisting of shallow-marine epeiric carbonates, especially dolostones, producing oil and gas from karst and unconformity traps. Two years ago I worked in the Algerian Sahara in the CambroOrdovician reservoirs of Hassi Messaoud. This hydrocarbon province contains one of the largest reservoirs in the world. It is drastically different from the setting in North America: the reservoirs are composed of sandstones with a porosity in the range of 15 to 2o% producing from non-marine facies, such as fluvial and take deposits and local deltaic settings. Oil production comes for the most part from sandstones and conglomerates of braided stream to alluvial facies. Reservoir quality is related to fractures and diagenetic porosity, especially through the dissolution of feldspar. I wished at the time I would have had this current manuscript of Dr. Monzer Makhous at my disposal. This extensive book presents in great detail every important aspect of the Sahara reservoirs which I studied, including regional geology, mineralogy, petrology, reservoir characteristics, especially the important secondary porosity, petrophysics, carbon isotopes, and migration and accumulation of hydrocarbons, and hydrocarbon generation and preservation. In the Sahara, as elsewhere, stratigraphic and diagenetic reservoirs are becoming the prospects of the future, and this book shows the reader how best to understand these reservoirs and explore for them. This study is most impressive. Dr. Makhows spent 17 years in the Research Center of SONATRACH evaluating zo ooo samples mineralogically and petrologically, and 5 ooo by chemical analysis. The book's ten chapters summarize the most detailed petroleum reservoir analysis I have seen in recent years. For those working in the Sahara this book is a gift. For those working elsewhere it should serve as a case history for their own areas of study. Dr. Gerald M. Friedman

Contents

Introduction

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1

1

Methods of Investigation

1.1 1.2

Analytical Work and Data Treated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Structure and Extent of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2

Characteristic Features of the Region: Geological Structure, Lithofacies, Paleogeography and Genetic Framework of Non-Structural Traps ........ 9

2.1

Geological and Structural Characteristics of the Saharan Platform and Outline of the Evolution of Its Sedimentary Basins 2.1.1 Sedimentary History and Stratigraphic Sequences . . . . . . . . . . . . . . . . . . . . . . . . Lithofacies and Sedimentary Environments of the Oil- and Gas-Bearing Formations of the Triassic Province 2.2.1 Paleozoic Sediments of the East Saharan Syndinorium (Ghadames and Illizi Basins) 2.2.2 Paleozoic Sediments of the Oued el-Mya Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Sedimentary Genesis of the Triassic Deposits with the Northwestern Part of the Triassic Province as an Example 2.2.4 Mineral Transformations in Sandy Reservoir Rocks Resulting from the Interaction Between Interstitial Waters and Primary Components During Early Diagenis: The Diagenetic Signatures ....... 2.2.5 Evolution of Reservoirs During Progressive Subsidence (Late Diagenesis): Influence of the Sedimentary Inheritance ........... 2.2.6 Triassic Sediments of the Oued el-Mya Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.7 Triassic Sediments of the Ghadames Basin and the Northern Flank of the Illizi Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geological Parameters for the Formation of Non-Structural Traps and Their Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Non-structural Traps and Methods of Investigation . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Lithostratigraphic Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Lithological Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Stratigraphic Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 Traps Related to Volcanic Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.6 Traps Resulting from Differential Compaction of Sediments ............ 2.3.7 Morphological Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,3.8 Diagenetic Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brief Petroleum Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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16 18 i8 25 29

37 39 42 44 46 47 49 51 54 58 58 59 61 62

X

Contents

3

Main Factors of Reservoir Compaction

3.1 3.2

Evolution of Density, Porosity and Permeability with Depth 63 Relationship Between Structure, Gravitational Compaction and Pressure Solution of Granular Reservoir Rocks 65 3.2.1 Gravitational Compaction and Pressure Solution . . . . . . . . . . . . . . . . . . . . . . . . 65 3.2.2 Structural Arrangement of Sandstones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Reservoir Compaction by Silicification from Other Sources . . . . . . . . . . . . . . . . . . . . . 75 3.3.1 from Underground Waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 3.3.2 Silicification Through Transformation of Feldspars, Detrital Illite and Other Micaceous Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 3.3-3 Silicification by Illitization of Smectite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Main Controls of the Compaction of Reservoir Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 3.4a Thickness of Sandy and Silty Reservoir Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 3.4.2 Argillaceous Diagenesis and Related Abnormal Formation Pressure ... 87 3.4.3 Early Development of Overgrowth Rims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.4.4 Presence of Mesozoic Evaporites 89 3.4.5 Temperature and Pressure 90 3.4.6 Authigenic Transformation of Argillaceous Cement . . . . . . . . . . . . . . . . . . . . . . 90 3.4.7 Early Invasion of the Reservoirs by Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . 96 Silica Solid Phase Transformation: A New Concept for Sandstone Diagenesis as Revealed in North African Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 3.5.1 Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.5.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 3.5.3 Results 101 3.5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 3.5.5 Solid Phase Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

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63

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S i l i c i f i c a t i o n

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34

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3.6

4

Reservoir Decompaction and Formation of Accumulation Capacity (in Secondary Porosity) of Reservoir Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Decompaction Due to Solution of Binding Compounds of Sandstones (Carbonates, Silicates, Aluminosilicates) 4.1.1 Relation in Time and Space (as a Function of Temperature) of Organic and Mineral Diagenesis After Formation of Reservoir Properties .... 4.1.2 Thermodynamic and Stoichiometric Regime of Formation of Carbonic and Organic Acids and Their Role in the Establishment of Secondary Porosity in Reservoir Rocks 4.1.3 Model of Differential Dissolution and Redistribution of Carbonate Cement with Compaction/Decompaction of Reservoirs in Space as Based on Carbon Isotope Data 4.2 Decompaction-Compaction by Intergranular Pressure Solution (of Quartz Grains) and Removal of SiO~ by Alkaline Solutions . . . . . . . . . . . . . . . 4.2.1 Pressure Solution and Quartz Cement 4.2.2 Factors Controlling Pressure Solution 4.2.3 Silica Budget 4.2.4 Mechanisms of Silica Transport 4.2.5 The Role of Pressure Solution in the Evolution of Porosity

131

4.1

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131 131

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149 159 161 162 166 170 173

XI

Contents

4.z.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Generalized Models of the Transformation of Oil-Bearing and Reservoir Formations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Model of Diagenesis in Space and Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Generalized Model for the Transformation of Reservoir Rocks, Mass Transfer and the Formation of Reservoir Properties .............

178

5

General Geochemical Features of Generation, Migration and Accumulation of Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

183

5.1

Geochemical Characterization of Potential Source Rocks, Hydrocarbons and Burial Histories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Silurian Graptolitic Source Clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5A.a Devonian Source Shales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Generation and Directions of Migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.a.1 Generation in the Silurian Source Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Generation in the Devonian Source Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geochemistry of the Triassic Province . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Source Rocks in the East of the Province (Ghadames and Illizi Basins) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Source Rocks in the North of the Province (Oued el-Mya and Triassic Basins) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3.3 Characterization of Petroleum in the Eastern Area of the Province ... 5.3-4 Petroleum Types and Their Variations in the North of the Province., 5-3-5 Conditions for Hydrocarbon Generation in the North of the Province .. 5.3.6 Conditions for Hydrocarbon Generation in the Eastern Area of the Paleozoic Province . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.7 Petroleum to Source Rock Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

193 197

Burial History and Kinetic Modeling for Hydrocarbon Generation

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207

The Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 The Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2 Burial and Thermal History Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Modeling Maturation History and Hydrocarbon Generation .......... 6.1.4 Additional Features of Our Burial and Thermal Modeling ............. 6.1.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Applying the Model to Saharan Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Geological Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . 2 . 2 0 u e d el-Mya Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Ghadames and Illizi Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Southern and Western Basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

207 207 209 22t 224 230 230 235 240 244 251 252

4.3

5.2

5.3

6 6.1

174 175 175

183 t83 184 185 185 186 186 186 188 190 191 192

7

Degree of Preservation of Hydrocarbon Accumulation as Indicated by Carbon Isotope Analysis

7,1 7.z

Methods Employed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 General Data on Carbon Isotope Composition of Sedimentary Rocks (Carbonates) and Organic Matter from Northeastern Algeria ................. 256

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255

xII

7-3

7.4

Contents 7.2.1 Carbon Isotope Composition of Carbonate Rocks . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Carbon Isotope Composition of Organic Matter . . . . . . . . . . . . . . . . . . . . . . . . . The M e c h a n i s m of Stable Carbon Isotope Fractionation (~2C vs. t3C) and Regularities in Their Distribution in Jurassic and Cretaceous Deposits of Northeastern Algeria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 D e c o m p o s i t i o n and Oxidation of Organic Matter . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Methanogenic F e r m e n t a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3 Sulfate Reduction by Bacterial Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

256 262

263 263 266 266 267

8

Reconstruction of Temperatures from Organic and Mineral Diagenetic Criteria

8.1

8.3

Reconstruction of Temperatures from Degree of Structural Ordering in Mixed-Layer Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystallographic Features of Clay Minerals as T h e r m a l Indicators in Petroleum Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary ................................................................................

271 277

9

General Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

279

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.2

10 Analytical Methods and Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Io.1 Inorganic Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lo.1.1 X-ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lO.1.2 Scanning Electron Microscope (SEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lO.1.3 Microanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lo.1.4 Major Element Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lo.1.5 Trace Element Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lo.L6 Infrared S p e c t r o m e t r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lo.1.7 Physical Measurements lo.2 Organic G e o c h e m i s t r y Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lO.2.1 Extractions and Separations 10.2.2 Vitrinite Reflectance Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . lO.2.3 Pyrolysis lo.2.4 Liquid C h r o m a t o g r a p h y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . to.2.5 Gas C h r o m a t o g r a p h y lO.2.6 Gas Chromatography-Mass Spectrometry (GC-MS) . . . . . . . . . . . . . . . . . . . . 10.2.7 Organic Carbon Isotopic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10,2,8 Mineral (Carbonate) Carbon Isotopic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . lo.2.9 C a t h o d o l u m i n e s c e n c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

269 269

283 283 283 284 284 284 285 285 286 287 287 288 288 288 289 289 289 290 291

References .............................................................................

293

Supplementary References

303

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Index .....................................................................................

309

Introduction

The Sahara contains two main hydrocarbon-bearing complexes (Fig. o.1), the older of which encompasses reservoirs in the Cambro-Ordovician and the younger one those in the Triassic. The hydrocarbon-bearing rocks have undergone secondary transformations under defined conditions to such an extent that their initial characteristics have completely disappeared. These transformations are characterized by two essential factors: the first one is the vertical inhomogeneity in the transformation of the rocks resulting from the vertical distribution of energy and matter. This is expressed by the succession of zones of compaction and decompaction which may be observed. The second factor tends to complicate the trends of the variation with depth. It is related to the particular history of development which may be subdivided into two parts. The distinction between these two stages, one during the Paleozoic and one in the Mesozoic, is highlighted by the intense Hercynian tectonic activity and orogeny followed by pronounced uplift and erosion. This has greatly lessened the heating of the rocks during the"thermal pause" in their geological history. The study presented here has made possible the elaboration and investigation of

the followingfundamental problems: t. reconstruction of the diagenetic history, unveiling of the mechanisms transforming the reservoirs which represents the key question in prospecting for and exploitation of hydrocarbon deposits; 2. outlining the new processes of decompaction of rocks at great depths by demonstrating the relative roles of the different processes at different depths. This entails the particularly important process of dissolution and transport of a tremendous amount of carbonate and silica cement under the influence of carbonic acid and in particular of organic acids like bifunctional carboxylic acids and phenols; 3. outlining the process of large-scale transport of silica under the specific conditions of the alkaline environment prevailing at great depths; 4- outlining non-structural traps formed by different mechanisms. The main aim was the characterization of the zones of decompaction and the outline of the mechanisms leading to the properties of the reservoirs. For this purpose it was necessary: - to investigate in detail the composition of the rocks with the aid of modern analytical methods and to treat the data obtained from a considerable number of samples taken from different basins in order to be representative of the different geological situations; - to design a number of models reflecting the characteristic diagenetic processes of the geological development of the basins which would allow us to establish

Introduction

z

general features of generation and migration of the hydrocarbons as well as the formation of the respective reservoirs. One of the most important rules in the formation of hydrocarbon deposits has been demonstrated, based on the fact that the primary migration of hydrocarbons follows closely on the formation of the secondary porosity, as during maturation of the organic matter the main phase of formation of the hydrocarbons takes place after the culmination of the decarboxylation which generates active acid solutions controlling the formation of the secondary porosity. This shows that the generation of hydrocar-

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Introduction

3

bons and the formation of the secondary (main) storage capacity of the reservoirs overlap in time and space. Because of this the direct links between the source of the hydrocarbons and the reservoirs in time and space favour the accumulation of hydrocarbons in reservoirs with secondary porosity. In this context the results of the present research are of great theoretical value for the study of the diagenetic transformations and of their influence on the formation of the trapping properties of the reservoirs. This leads to the establishment of new criteria for prospecting for hydrocarbon deposits, such as: 1. redistribution within the reservoirs of carbonate cement resulting from dissolution by acid solutions in diagenetically more mature reservoirs and its redeposition in overlying more mature zones. This leads to an increase in the porosity in the zones of dissolution and to a blocking of the reservoirs in the zones of precipitation; 2. correlation of the carbon isotopic composition (6'3C) of the five oil and bitumen fractions with the aim of establishing the sources of hydrocarbon formation in order to delineate which reservoirs under these conditions in the many possible sources would be the most promising targets of exploration; 3. evolution of the secondary migration of hydrocarbons by studying the mineral transformations along their routes of migration on the surface and at depth and in particular the carbon isotope composition in the carbonates and the organic matter. This approach will enable us to evaluate the degree of conservation of the hydrocarbon accumulations which underwent total or partial destruction in absence of a real cover, in particular under the conditions prevailing in certain tectonic complexes to the north of the Saharan Platform.

Practical Aspects of this Research 1. The maximum depth at which the secondary porosity resulting from mesokatagenetic decarboxylation may be preserved is more than two times larger than that at which the primary porosity may be preserved in sandstones of high mineralogical stability and more than three times larger than that in sandstones of low mineralogical stability. 2. We have tried to delineate the main zones of formation of non-structural traps which are promising, as undrilled promising anticlinal structures become rare. 3. The correlation of the carbon isotope composition (61~C) of the oil and bitumen fractions of all eventual source rock "conditions", in view of the large variety of the latter, allows us to point out the sources of the hydrocarbons which supplied the one or other reservoir and thereby to better direct the prospecting activities. 4- The chemical and kinematic simulation of the hydrocarbon-bearing basins enables us to reconstruct their burial history, the changes in the thermal regime of the sedimentary series as well as formation and migration of hydrocarbons in each oilbearing horizon. The innovation and perfecting aspect of our simulation in comparison with the other available models lies in the application of alternative methods for calculating the tectonic subsidence of the basins, the integration of the maturation of hydrocarbons as a function of burial as well as an earlier estimate of the overall potential of

4

Introduction

source rocks. For this purpose the algorithm for selecting the kinetic parameters of the reactions of hydrocarbon formation takes into account the history of maturation of the source rocks in time and space including the geological stage of maturation as well as the stage of pyrolysis of the source rock sample in the Rock-Eval system. If we omit from the calculation the geological stage, the energy spectrum of activation for the reactions is shifted abruptly- to higher values, leading to an underestimate of the hydrocarbon potential by one order of magnitude or more throughout the geological history.

Chapter 1

Methods of Investigation

The main concepts of this investigation are based on theoretical and experimental research which the author has carried out during his 17 years at the Centre of Research and Development of Messrs. SONATRACH in the fields of exploration and exploitation of hydrocarbon deposits. Particular attention was paid to the detailed study of the mineralogical and organic composition of hydrocarbon-bearing rocks, as well as of the hydrocarbons themselves and of the formation waters. For this purpose modern analytical techniques had to be employed: scanning and transmission electron microscopy (SEM and respectively TEM), SEM-image analyses, XRD, ICP, XRT, WDS and EDS microanalysis, IRS, mass spectrometry, chromatography, pyrolysis, etc. The petrographic characteristics of the reservoirs in rocks were determined in addition to the more classical methods by image analysis, permitting a characterization of geometry and structure of the pore space. The statistical treatment and graphic presentation of the enormous amount of the analytical data required a great amount of computer work. Virtually all geological problems studied here were dealt with on the basis of measured and calculated physico-chemical parameters: 1. Paleographic reconstructions were done by using, in addition to widely known geological criteria, parameters like the adsorbed complex of the clay minerals, their ion exchange capacity, their coefficient of alkalinity and the type of iron minerals present. 2. The diagenetic processes were delineated by using the structural and crystallochemical peculiarities of the clay minerals, including a calculation of the parameters of the crystal structure and the determination of the structural modifications of the various minerals. A detailed study of the mineralogy and crystallography of quartz, silica, and carbonates was carried out together with an analysis of the isotopic composition of the carbon. 3. In order to study the process of generation and migration of the hydrocarbons, one needs data on their phase composition, the mineral transformations associated with the transformation of the organic matter and the carbon isotope composition of the carbonates, bitumens and oils. 4. When carrying out the chemical and kinetic simulation of generation and migration of hydrocarbons, we have conducted experimentally an open pyrolysis at different heating regimes, taking into account the maturation of the organic matter with depth of burial of the source rock and applying other independent geothermal parameters.

6

Chapter 1 • Methods o f Investigation

1.1

Analytical Work and Data Treated The thesis is based on the results obtained by the author during his field and laboratory work while working on ten large hydrocarbon-bearing basins of the Saharan Platform. It deals with and evaluates an enormous amount oftithotogicai and geochemical data accumulated in the research centre of Messrs. SONATRACH between 1975 and 1992: the majority of this work was carried out by the author himself or under his guidance. Over lO ooo samples of sandstones and shales were subjected to integrated mineralogical analysis and analysed by XRF and SEM. Chemical analysis was carried out on more than 5 ooo samples. The petrophysical data (porosity, permeability, water saturation and density) were determined for sand- and siltstones at intervals of z5-5o m and the overall length of drii1 core studied was at least 3o ooo m. Special investigations of structure and geometry of the pore space with the aid of cathodoluminescence and SEM-image analysis were carried out on about 7oo sandstone samples. All petrographic, mineralogical and petrophysical data were compiled for each borehole in about 2o0 laboratory logs. Particular investigations like determination of the adsorbed complex, of ion exchange capacity, of coefficient of alkalinity and of type of iron-bearing minerals in combination with the mineralogy of the respective argillaceous horizons were carried out for more than 15o sections or respectively bore holes. The carbon isotope composition (613C) was performed on the carbonate cement of 18o sandstones, on the five oil and bitumen fractions in 21 source rocks and on some 50 oils. Additionally, when studying the secondary migration of hydrocarbons, the carbon isotope composition was analyzed on 15o carbonate samples and 4o bitumen samples taken on the surface. The author also made use of the results of pyrolysis of more than 2 ooo source rock samples and of the chemical and bituminological analyses of the organic matter in more than 7 ooo shales. He had at his disposal also microspectrophotometric determinations of the catagenic level of the organic matter in about 2 400 units, about 1700 chromatograms of the gaseous and liquid phases and complete chemical analyses of formation waters from about 1zoo horizons, including their mineral and organic compounds. Paleogeographic reconstructions of the sedimentary conditions were produced for many of the stratigraphic complexes of the main basins, widely using mineralogical and chemical data and in particular those on argillaceous rocks. Cathodoluminescence was greatly used for studying quartz diagenesis and for estimating the silica budget. It covered some 400 sandstone samples. 1.2

Structure and Extent of Work The first part of this book (Chapter z) deals with structures and geological history, paleogeography, the general features of development of the sedimentary basins and with the mechanisms forming non-structural traps. The second part (Chapters 3 and 4) describes the main factors of compaction-decompaction of reservoirs in the context of the diagenetic transformations and their characteristics in relation to their properties of accumulation and infiltration. The third part (Chapters 5 and 6) evaluates the geochemical peculiarities of generation, migration and accumulation of hydrocarbons

1.2. Structure and Extent of Work

7

on the basis of classical geochemical data and those resulting from chemical and kinetic modeling. The fourth part deals with specific applied aspects of isotope geochemistry for evaluation of the degree of preservation of hydrocarbon accumulation (Chapter 7) and temperature reconstruction from clay mineral and organic diagenetic criteria (Chapter 8).

Chapter 2

Characteristic Features of the Region: Geological Structure, Lithofacies, Paleogeography and Genetic Framework of Non-Structural Traps

2.1

Geological and Structural Characteristics of the Saharan a n d O u t l i n e o f t h e E v o l u t i o n o f Its S e d i m e n t a r y Basins

Platform

On the Saharan Platform, starting from the Hoggar Massif in the south, we observe a number of north-south running uplifts: (Amguid el-Biod-Hassi Messaoud, Tikhemboka-Zarza~'tine-Atrar, tdjeran-M'Zab, etc.) which are separated from each other by wide synclinoria, with the same trend underlying a number of depressions. Thus the East Saharan synclinorium may be subdivided into two depressions, the Illizi to the south and the Great Eastern Erg (or Ghadames) in the north. The Central Saharan synclinorium is also underlying two depressions, the Mouydir to the south and the Oued el-Mya to the north. On the West Saharan synclinorium we find the depressions of Ahnet, Reggane, Tindouf, Timimoune de Bechar, etc. (Figs. ~.~ and 2.2). These basins were subdivided by Guiraud et al. 0987) into North and South Saharan and a de-

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2.3 • Geological Parameters for the Formation of Non-Structural Traps

5:

of T-: are argillaceous and compacted. A similar phenomenon is encountered at Kef el-Argoub (KG): the sandstones of T-: produced oil in KG-2 and KG-4 whereas the same sandstones become clearly argillaceous in HLJ-z in structurally more favourable position and the T-: sandstones are non-productive at Hassi Ladjouad. These phenomena represent encouraging signs for the search for non-structural and structural traps.

2.3.3 Lithologicai Traps In the upper argillaceous-arenaceous Triassic (TAGS) of the Triassic Province two zones probably containing non-structural traps are normally delineated in the border areas of this region. The first zone occurs in the southern part of the Ghadames Depression in the vicinity of the Illizi Basin (see Fig. 2.:5, Zemlet Mederba area (ZM)). It extends with a general E-W trend over about :1o km along the inner margin of the wedging-out zone of the reservoirs. This region is characterized by a rapid rise of the sediments towards the south and the entire sedimentary complex of the Lower Mesozoic is wedging out rather abruptly to become replaced by the thinner Zarza'itine Series. From a "petroleum" point-of-view this zone is of interest because of important indications of oil, gas and water encountered in a number of holes (SOH% EOS-:, ZM-1) in the south of the Ghadames Depression. It is obvious that the lithological traps related to the wedging out of the reservoir rocks upslope could form in different parts of this zone. The second, about 70 km long zone is restricted to the southeast of the study area along the regional fault, i.e. to the Sedoukhane (SED, SEDE) and Dimnet (DIMN) regions (Fig. 2.:5). In plan this zone coincides with the analogous zone within the Lower Triassic but overlies it. From a petroleum point-of-view this zone is highlighted by the same factors. The regional fault borders the western, poorly studied limb of the Tartrat fold and a series of fields, in which economic quantities of oil and gas have been produced from the upper argillaceous-arenaceous Triassic lies on the eastern limb of this fold. Drill hole data show a thickening of the productive sandstones towards the west in the axial direction of this fold. It is highly probable that favourable reservoirs could have formed also on the western limb of this fold. These observations favour the formation of non-structural traps as a result of wedging-out and erosion of the reservoir beds in the upslope direction. Another zone, located in the western parts of the Triassic Province, exhibits signs of lithological traps. Drill hole data had already revealed some time ago an approximate line of regional wedging of the reservoir rocks of the argillaceous-arenareous Triassic (TAG). This boundary passes throughout the northern part of the Oued elMya Depression, forming a wide arc with the convex side pointing to the south. It may be traced between drill holes HRS-: and CHA-: over the southern closure of the Tilhremt rise (Fig. 2.14). Then it runs southward to the interior of the depression in the area of hole DR-: to turn to the northeast, passing along to the western slope of the Hassi Messaoud dome between holes GEC-1 and GEB-:. The boundary is probably sinous. South of the limit of extension of the reservoir rocks of the TAG the succession is replaced completely by impermeable argillaceous formations with a thickness not exceeding :oo m. The entire wedging-ont of the argillaceous sediments of the corn-

52

Chapter 2 - Characteristic Features of the Region

o i~

~

~

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%

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5o mD is not large, amounting to 1.o-1.5% when the pressure increases from 3ao to 5oo kg cm-L We have thus to concede that because of petrographic analogy at depths of 4.o-4.5 k m the reservoir rocks do not lose much permeability when the rock temperatures increase only little. It is obvious that the mechanical compaction is also a function of the duration of the influence exerted by the effective pressure on the framework of the rocks. Table 3-3 presents the characteristics of rocks comparable in granulometry and percentage of argillaceous cement, illustrating the extent of quartz solution under pressure. To judge from the importance of sutured intergranular contacts, the younger Triassic and Jurassic rocks are less compacted than those of the Cambrian. Two other factors, viz. depth and petrographic composition, will also exert a notable influence, but it will be very difficult to evaluate them quantitatively without special investigations. It could

3.2 - Relationship Between Structure, Gravitational Compaction and Pressure Solution

69

Table 3°3. Correlation between age, day content and compaction level in various oil fields

Field

Age of rocks

Mean depth (m)

Proportion of sutured intergranular contacts (%) without with overovergrowth growth

total

Proportion ofargillaceous cement (%)

Mean size of grain (mm)

No. of observations

Hassi R'Mel

Triassic

21 O0

41

3

44

15

0.17

3t

Hassi

Jurassic Cambrian

1 900 3500

50 54

0 8

50 62

17 21

0.15 0.18

20 42

Messaoud

be assumed, e.g., that the greater compaction of the Jurassic rocks compared to those of the Triassic is caused by the higher content of ductile argillaceous debris in the former (Table 3.3, Fig. 3.4). Because of this, in the search for reservoir rocks possessing the best potential in sequences intensely transformed during categenesis one should take into account their mineralogical composition and in particular the content of quartz and clay. Experience from studies in polymict reservoir rocks containing 20-35% ductile debris shows that their accumulation and filtration Characteristic drop sharply at depths below 4 500-5 ooo m. It has to be pointed out that mechanical compaction of rocks differing in grain size will lead to variable losses of their reservoir characteristics with time. In fine-grained rocks the pores, because of their small dimensions, are rapidly reduced during interpenetration of the grains, explaining the accelerated rate of loss in reservoir properties. Because of this the search for better reservoir rocks in sequences undergoing more intense diagenesis should concentrate on coarse-grained horizons as these will maintain open pores over a longer time because of the larger dimensions of the latter. It is obvious that the formation of overgrowth rims will make the pores and the channels narrower. From the data discussed above (Table 3.2, Fig. 3-3) we can evaluate the influence exerted by overgrowth on the pore dimensions. The overgrowth rims tend to become enlarged in coarser-grained rocks and we have to assume that the process of grain penetration is followed by the formation of overgrowth rims. Considering the shrinkage of the pores resulting from compaction and overgrowth we have determined graphically how this process would affect the volume of the pores if overgrowth would encircle all grains bordering the respective pore. The results are shown in the last column of Table 3.2. They illustrate that when the above-mentioned conditions are met and the pressure solution of quartz operates at a medium rate the greater part of the pores will be entirely eliminated by their processes in fine- to mediumgrained sandstones. In contrast to this in rocks with a mean pore size above 0.5 mm the pores will not close up completely and their dimensions will actually grow in coarse-grained rocks. It is obvious that these are only rough approximations, but they are clear evidence of the fact that coarse-grained sandstones and gravel beds represent the best reservoir rocks in successions undergoing intense diagenesis. The calculations performed also show that fine-grained rocks are the first ones to be compacted by the process of quartz crystallization. This process is thus regular and any difference in timing will result from the differences in rock types encountered.

70

Chapter 3 . Main Factors of Reservoir Compaction

As a first approximation the variations in accumulation and filtration properties of the reservoir rocks down in a section that has undergone early diagenesis depend largely on the main characteristics established during sedimentation. The best reservoir rocks will form on the shallow continental platform and their good filtration properties result primarily from their low content in argillaceous cement. This question

Fig. 3.4. G r o u p s o f r e s e r v o i r

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3.2 • Relationship Between Structure, Gravitational Compaction and Pressure Solution 100

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50

O u

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10

20

3'0

Porosity (%)

Fig. 3.5. Correlation diagram of porosity with mineralogical (chemical) maturity (increase of silica, especially secondary) and with structural maturity related to different facies-environment situations

has been intensely studied with the aim of establishing a quantitative link between the tithofacies conditions of the reservoir rocks and their accumulation and filtration properties under the same diagenetic regimes. The correlation plot established for reservoir rocks formed in different environments in different basins (Fig. 3.5) shows that porosity grows with increasing facies energy, all other conditions remaining unchanged. This increase in sandstone porosity takes place in the following sequence: tidal bars > delta > fluvial > marine. The succession is justified from the point of view of textural maturity of the reservoir rocks as evident from the high degree of sorting and rounding of the primary detrital material (mostly quartz) in the same order. In other words, under identical diagenetic conditions as expressed by the same amount of total and secondary quartz porosity, will be preserved best in tidal bar sandstones, followed by deltaic sandstones and finally in fluvial and marine sandstones. 3.2.2 Structural Arrangement of Sandstones

In order to obtain a more representative estimate of the compaction of reservoirs we have studied the contact index calculated as an average value of straight, concave/convex and sutured contacts. Sandstones containing much ductile cement suffered more intense compaction. Independent of their proportion of ductile (argillaceous) cement the Triassic reservoirs have suffered only weak compaction with depth (Fig. 3.6a, c). Their grains were displaced by shifting of one grain against the next at the contact points and by rotation (mechanical rearrangement).

72

Chapter 3 • M a i n Factors o f Reservoir C o m p a c t i o n

100%tangentialcontacts

100%embayedcontacts

100%long contacts 200 0 10 i

210

220

230

240

250

260

270

280

5

10

15

20

25

30

35

40

8 '

6 '

4

2

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.OK20 (%)

2

2.5'

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290Density (gcm "3) 45Montmorillo. nite in interlayered(M-I) minerals (%)

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A

t

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-

: :"

4.0.

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I? 22 Porosity (%)

2'6

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34

36

Fig. 3.6a,b. Mechanical compaction features of Saharan reservoirs, a Contact type plot for sandstones from s Cambro-Ordovician, Hassi Messaoud oil field (cement content x > o.75, the above-mentioned transformation should result in 2% secondary silica in the total rock. Such an amount of primary illite, however, could not have been contained in the sandstones of the study area and there is frequently even detrital kaolinite in addition to the newly formed one. Judging from the mineral composition of the shales in which such secondary transformations would be reduced to a minimum there could have been only 15-3o% detrital illite as a primary constituent in the sandstones of the Hassi Messaoud field whereas the amount of illite detected in the Cambrian siltstones is 3o-5o%. Consequently, only 1% of secondary quartz could have resulted from the transformation of illite to kaolinite. Thus, widely developed kaolinization of illite and feldspars is accompanied by the liberation of silica in the form of silicic acid which is mostly consumed in the formation of overgrowth on quartz grains. The extent of this silicification, however, is controlled by other factors: the chemical nature of the environment created and especially the permeability of the rocks. The lower the permeability, the more intense silicification wilt be. The same origin of silica is frequently assigned to the recrystaltization of detrital micas in an alkaline environment. The recrystaltization of allogenic micas (or hydromicas), muscovite and biotite results in the liberation of SiO2 whereas the alkaline environment created by this also favours some degree of leaching of the quartz grains. In rocks of low permeability even this dissolved silica starts becoming deposited on neighbouring grain as overgrowth rims because of its low mobility in the pore space. A characteristic feature of this redeposition of secondary silica resulting from the transformation of clayey matter and feldspars is the discontinuous nature of these rims. At places the quartz grains are transformed into monolithic aggregates reminiscent of a quartzitic texture (Plates 1, 2) and one can make out welt-developed crystal faces. However, in addition to the overgrowth rims there are small pyramidal quartz crystals oriented at right angles to the surface of the grains on which they develop. Under the SEM (Plate 2) the quartz grains exhibit an elongate shape and are frequently oriented parallel to the stratification of the beds. There are also overgrowth rims with straight outlines completely enveloping the detrital quartz grains. There is even a well discernible second generation of secondary silica especially in sandstones cemented by carbonates with a poikilitic texture. This generation is characterized by particularly regular crystal faces on the quartz grains (Plate 2). The formation of the different overgrowth rims is here linked to the recrystallization of the carbonate cement

80

Chapter 3 - Main Factors of Reservoir Compaction

Plate 1. Morphological features of quartz, a, b Monolithic quartz aggregates; c progressive crystal growth and formation of growth rims on quartz; d, e discontinuous growth rims on quartz; f welldeveloped crystal faces on quartz (/eft)

Plate 2. Morphological features of quartz, a Discontinuous growth rims on quartz; b, h quartz crys- i~ tals grown and oriented parallel to stratification; c, d quartz crystals grown and oriented at right angles to the surface on which they developed; e, f several generations of growth rims on quartz grains; g, h particularly regular crystal faces on quartz which frequently are encountered in carbonate-cemented sandstone

3.3 • Reservoir Compaction by Silicification from Other Sources

81

8z

Chapter 3 . Main Factors of Reservoir Compaction

(siderite, dolomite and calcite). The mass of the carbonate cemem is enriched in free silica furnished by the corrosion of the quartz grains and of other detrital compounds as well as from the transformation of the initial argillaceous cement. In other words, during recrystallization of the carbonate on compaction (or decompaction) the calcite crystals grow individually. A poikilitic texture is formed and the primary calcic mass clears itself of impurities and of the silica, thereby supporting the mineralization and reconstitution of the crystal faces of the quartzes. This process is accompanied by an opening of the pore space and consequently the overgrowth on the quartz grains is no longer limited in volume and is able to advance until crystal faces appear. 3.3.3 Silicification by lllitization of Smectite Illitization of smectic and of mixed-layer minerals takes place under liberation of some silica. The mixed-layer minerals must have existed in large quantities in the reservoir rocks studied, a fact that is confirmed by their occurrence in younger or diagenetically less altered strata of all basins. The process started during the early phases of diagenesis and is controlled initially by the thermal regime and then by pressure. Silica liberated here is deposited on detrital quartz grains in the pore spaces, representing one of the sources of secondary silica as a result of the presence of large quantities of mixedlayer compounds in the initial argillaceous cement. The transformation of smectite into illite accompanied by the liberation of silica takes place in argillaceous strata in direct connection with the diagenesis of organic matter. The organic acids and the carbonic acid set free play an important role in the extent of mobilization and distribution of the liberated silica. Quantitative estimates show that the silica liberated during the transformation of mixed-layer components of the illite/smectic type or of smectite alone into illite may a m o u n t to 2.2 g SiO 2 per loo g of clay (Towe 1962). Clayey components account for up to 6o% of the sediments (argillaceous siltstones). Even if the entire clayey matter was composed of mixedqayer minerals and if under this hypothesis all the silica liberated was transported towards the reservoir rocks, only 1.3% of quartz or secondary silica would be deposited (Towe 1962). The development of dioctahedral illites, characterized by elongate and lamellar shapes of the newly formed crystals, could perhaps be considered as indicator of the above-mentioned process. The older authigenic itlites may be distinguished by their more elongate crystals and in particular by their rightangle orientation against the bedding planes, i.e. in the b-axis (Plate 3). The illite formed from the mixed-layer components of the illite smectite (I/S) type mostly exhibits fibrous growth (Plates 3, 4) whereas the illites formed by transformation of kaolinite are characterized by a lamellar habitus (Plate 4). On the whole, the clay minerals with an elongate crystal habitus make up a continuous sequence of mixed-layer minerals of the smectite-illite type with a dioctahedral structure between the two extremes smectite and respectively iltite of the sericite type.

Plate 3. Development of authigenic illite, a Chlorite; b-d development of elongate and lamellar crys- 1~ tals at right angles to the stratification (b-axis); e - h authigenic illite with fibrous crystal habit developed by transition from I/S mixed-layer minerals

3.3 • Reservoir Compaction by Silicification from Other Sources

83

84

Chapter 3 Main Factorsof ReservoirCompaction

3.4, Main Controls of the Compaction of Reservoir Rocks

85

3.4

Main Controls of the Compaction of Reservoir Rocks Compaction due to increasing geostatic pressure, aside from possible tectonic compression, leads to a large reduction in porosity and one can note a priori an increase in density. The loss in porosity resulting from displacement and mutual approach of the grains, from the deformation of plastic grains and from pressure solution, to some extent is compensated by the formation of secondary diagenetic porosity. Not considering fracturing, the secondary porosity is formed by the partial or total dissolution of the matrix components between the grains and other debris, as well as of the cement of the respective sandstones. Where fracture porosity is developed like, e.g., in the Hassi Messaoud field, it may account for up to 4o% of the total porosity. The secondary porosity is formed virtually all along a section and its development does not abruptly stop at a certain depth. This secondary porosity accounts for the greater part of the total porosity over the depth interval from 2.6 to 3.8 km. The modal values of secondary porosity in the Cambro-Ordovician reservoir rocks of Oued el-Mya range between 25 and 5o% of the total porosity. Secondary porosity makes up an important fraction of the total porosity even in reservoirs now lying at lesser depths. This situation is well developed in the Paleozoic reservoirs of the Ahnet-MouydirGourara basins which have been uplifted to depths of o.6-z.7 km by the Hercynian orogeny. Naturally, secondary porosity would have formed when the region considered was at greater depths than now, but it was preserved after uplift. Nevertheless, the gradients of porosity and density vary within the same region. Such variations are particularly characteristic of the Iltizi Basin and the Ahnet-Mouydir-Gourara Basins. In addition to other factors like facies types, this observation is obviously an expression of variations in geothermal gradients.

3.4.1 Thickness of Sandy and Silty Reservoir Rocks The preservation of initially porous reservoir rocks especially at greater depths is above all a function of the thickness of the succession. The greater this is, the smaller will be the effect of compaction (Fig. 3.1o). This phenomenon has been noted in numerous reservoirs studied and is encountered in different geological contexts. Furthermore, within a certain layer the porosity will increase from the top and the bottom towards its centre (Fig. 3.11). The statistical treatment of the respective data (Figs. 3.1o,3.11) shows for different geological complexes in various basins of the Saharan Platform that: 1. the relative growth rates of porosity, i.e. the preservation of the accumulation potential of originally porous reservoir rocks, as a function of depth are more sensitive than the growth rates of permeability (Fig. 3.1o). Neglecting the other factors of compaction this phenomenon may be explained by the fact that permeability depends primarily on structure and geometry of the pore space;

•~ Plate 4. Developmentof authigenic illite, a-d Illitization of I/S mixed-layerminerals. Note fibrous habitus of illite; e-h developmentof authigenic lamellar iltite crystals from kaolinite

86

Chapter 3 -

I

II

,

III

Sandstonesfrom: ----I Ahnet-Mouydir GouraraBasin(PZ) II lllizi Basin(D+C) . . . . Ill Oued-My --x--~ tV GhadamE

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Main Factors of Reservoir Compaction

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Fieldof[hangefor sandstones from(PZ),Oued-MyaBasin Fieldof changeforsandstones from(D),GhadarnesBasin

Fig. 3.10. Curves of relative change of porosity and permeability with thickness of beds (based on average statistical data). I Sandstones from Ahnet-Mouydir-Gourara Basin (PZ), // sandstones from Iltizi Basin (D + C), lII sandstones from Oued el-Mya Basin (PZ), IVsandstones from Ghadames Basin (D) Distance from center to bed roof (In)

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./ '"* J 20 Distance from center to bed bottom (in)

Fig. 3.11, Curves of relative change of porosity and permeability within sandstone beds (based on average statistical data). I Sandstones from Ahnet-Mouydir-Gnurara Basin (PZ), II sandstones from Illizi Basin (D + C), HI sandstones from Oued el-Mya Basin (PZ), I V sandstones from Ghadames Basin (D)

3.4 • Main Controls of the Compaction of Reservoir Rocks

87

2. the relative growth rates of porosity and permeability as a function of the bed thickness in the Paleozoic reservoir rocks in the Ahnet-Mouydir and Gourara basins are lower than those in the Paleozoic reservoirs of the Oued el-Mya and Ghadames Basins (Fig. 3.11). This situation is in perfect agreement with all diagenetic features of compaction observed in the various basins. It goes without saying that the anomalous thermal flux in the former group of basins may lead to a reduction in the stability of the grains and the matrix of the sandstones and to an intensification within them of the processes of chemical compaction and pressure solution.

3.4.2 Argillaceous Diagenesis and Related Abnormal Formation Pressure Not considering other factors the preservation of porous reservoir rocks at greater depths in Paleozoic and Triassic reservoir rocks is explained by the appearance of an abnormally high formation pressure which acts as an absorber reducing the effective constraints suffered by these rocks. In the Oued el-Mya and Ghadames Basins and to a lesser extent in the Illizi the Silurian, Devonian and Triassic shales contain a large amount of mixed-layer days of the smectite-illite type, accounting for up to 2o-25% in many regions. It appears that the development of an abnormally high formation pressure in the deeply buried sediments results from a rapid compaction of the silty-argillaceous and sandy rocks in geosyndinal basins (Bradley 1975; Hower et al. 1976). This is caused by the very rapid accumulation of sedimentary material followed by the burial as characteristic of the Triassic Province. At temperatures around loo °C the mixed-layer minerals of the smectite-illite type react with the potassium of the feldspars associated with them and with the interfoliar water when the illitic component increases in the mixedlayer minerals (Boles and Frank 1979; Huang et al. 1993). The detailed correlation between porosity of the sandstones, density of the associated shales, proportion of smectite and illite in the mixed-layer compounds and potassium content of the shales with depth in the Paleozoic of the Oued el-Mya Basin exhibits a concordant behaviour of these parameters (Fig. 3.1z). As already mentioned, a certain decompaction takes place at the depth of z.8-3.8 km, corresponding to mesodiagenesis, within the reservoir rocks because of the development of secondary porosity. In this interval even the density indices vary only very little and sometimes decrease only weakly, suggestive of the appearance of a zone of abnormally high formation pressures. The increase of the illitic compounds in the mixed-layer clay minerals with depth is accompanied in the shales by an increase in the K20-content coming from the feldspars and the interfoliar water. The transformation of smectite into illite liberates much water from the clay structure in a number of successive stages which depend on the rate of temperature and pressure variations, i.e. on the rate of subsidence (Burst 1976). These variations may lead to a volume increase of the silty-argillaceous horizons and to the stabilization or reduction of the density of the rocks observed at depths somewhat below 3.o km in the shallowmarine formations, in particular in the marine Devonian and Carboniferous sequences of the Illizi Basin and in the deltaic rocks of the Lower Paleozoic in the Oued el-Mya Basin. An enormous amount of water is also liberated during the diagenesis of the Triassic and Jurassic evaporites. These waters are accumulated in the adjacent permeable sandstones which prove to be under abnormally elevated formation pressure. We must understand that the excess pressure resulting from the liberation of water during

88

Chapter 3 • Main Factors of Reservoir Compaction 2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9 Density (g cm-3)

0

5

10

15

20

25

30

35

40

10

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0

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45 Montmorillonite in interlayered (M-I) minerals (%)

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6

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26

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36

Fig. 3.12. Correlation curves of 1 reservoir porosity,2 adjacent shales density,3 montmorillonitelillite proportion in adjacent shales, 4 potassium content in shales, with depth for Triassic and Palaeozoic sediments, Oued el-Mya Basin

diagenesis of the shales and the decompaction of the sandstones because of the development of secondary diagenetic porosity may be superimposed onto each other in time and space. This is the key phenomenon in understanding the migration of the hydrocarbons and their accumulation in the traps of the Saharan basins. One of the essential factors for the appearance of abnormally high formation pressures in this province lies in the fact that during the Mesozoic thick layers of sediment were deposited and buried at rates exceeding the ability of the entrapped waters to adapt to the growing overlying weight. Another factor controlling the formation of an excess pressure would be tectonic activity. The development of an abnormally high formation pressure requires an impermeable cover sequence. It is clear that this cover has been affected in several regions, but in particular in the Illizi Basin and the basins in the southwestern Saharan Platform, by fractures and other features leading to the re-establishment of normal formation pressures. This suggests that most of the zones mentioned above as exhibiting excess formation pressures will be young in age, probably belonging to the Cenozoic.

3.4 - Main Controls of the Compaction of Reservoir Rocks

89

3.4.3 Early Development of Overgrowth Rims One of the main factors counteracting compaction in the quartz sandstones is the formation, prior to the important Mesozoic subsidence of the sediments, of a quartzose matrix up to incomplete occlusion in the form of overgrowth rims. These quartz rims represent a solid matrix opposing further consolidation of the rocks at great depth. This phenomenon is one of the main factors facilitating the conservation of good reservoir properties at greater depth in all basins studied and especially at Oued el-Mya and Ghadames. The Hassi Messaoud field is a well-known example in which, in addition to leaching initiated during the Hercynian orogeny, the good reservoir properties result from the early formation of quartz overgrowth rims up to incomplete occlusion prior to the Mesozoic subsidence. These rims strongly countered the subsequent compaction of the reservoirs. The other geological factors remaining unchanged, early diagenetic formation of a solid skeleton may be recommended as a criterion in the search for reservoir rocks little compacted at greater depths in the Saharan basins.

3.4.4 Presence of Mesozoic Evaporites The Saharan Platform is a geological structure with a double-level setup: sedimentary formations of the Paleozoic and Mesozoic separated from each other by a long interruption in sedimentation and a deep erosion in connection with the Hercynian orogeny, each level possessing its own rules for compaction and decompaction of the reservoirs. The Mesozoic evaporitic formations, and especially the salts like halite, play a double role in the geological history of the province. At first, these formations create a magnificent regional cover for the reservoirs of the Lower Triassic and sometimes of the Paleozoic. Then the weak thermal isolation lowers the heating of the sediments and the degree of diagenetic transformations and compaction, i.e. they contribute to the preservation of excellent properties in the underlying reservoirs. The sediments underlying the salt-bearing formations are continental arenaceous-argillaceous sediments, carbonates and sulfates. The reservoirs in the Triassic sandstones, in particular in the lower part of the succession, possess good reservoir characteristics which, in addition to other geological factors, may be attributed to the low degree of heating and to the moderate catagenesis of the sediments as a result of the weak isolation by the overlying thick beds of Triassic and Jurassic salts. Finally, the relatively moderate compaction of the Triassic and sometimes of the Paleozoic reservoirs is attributed, in addition to other factors, to the low density of the saks and to the presence of a low geostatic load. This circumstance may be considered as one of the reasons explaining the existence of Devonian and Cambro-Ordovician sandstones with a porosity in the range of 15-2o% at a depth of 3.4-4.z km in the Triassic Province. Such porosities are encountered outside the Triassic Province only at depths of 2.z-2.6 kin. In addition to the favourable role played by the salt-bearing formations, the lowering of the degree of compaction in the reservoirs may also be ascribed to abnormally high formation pressure, to the development of secondary porosity and to the formation of overgrowth rims on quartz prior to the Mesozoic subsidence.

90

Chapter 3 • Main Factors of Reservoir Compaction

3.4.5 Temperature and Pressure Studies of the evolution of temperature and pressure gradient with depth allow us to appreciate the role of these two factors. The geothermal gradient of the Saharan basins ranges from 0.7 °C / loo m in the Triassic sediments to 3.5-6.0 °C / lOO m in the Paleozoic rocks, in particular in the basins of the south and southwest. The gradient is generally moderate in the Oued el-Mya and Ghadames Depressions at 1.8-3.4 °C / lOO m, but rather variable in the Illizi Basin with 2.0-4.2 °C / lOO m. The temperature gradient, however, fluctuates considerably within the same region. The lowest gradient is encountered in the salt-bearing parts of the section. The value itself is a function of the thermal conductivity and of the rate of thermal flux. It is obvious that the thermobaric fluctuations depend on the variations of the other physical properties of the rocks with depth. The Cambro-Ordovician reservoir rocks of the Oued el-/vlya Basin with a geothermal gradient of 3.8-4.5 °C / loo m encompass medium-grained reservoir rocks with a porosity of 8-1o%. In the Ghadames Basin, however, the geothermal gradient in the Devonian sandstones is 2.8-3.1 °C / loo m and their porosity about 16%. A gradient of 1.9 °C / lOO m is associated with a porosity of 22% in the Upper Paleozoic sandstones of the same basin. It has to be kept in mind that the spontaneous modification of the pore space by thermal expansion or by compression of the rock matrix will not change the porosity index to a great extent. However, the indirect influence of the temperature on the porosity manifests itself by the velocity of the chemical reactions leading to the compaction of the rocks by cementation. The pressure exerted on the rocks will alter their physical properties, its influence varying with type and structure of the rock in question. There is always a limitation for the compressibility of a rock at the fracturing limit of its mineral components, but in the Saharan basins we very rarely observe crushed grains even at depths below 4 kin. This is explained by the fact that a certain portion of the purely physical transformation is replaced essentially by chemical processes especially during pressure solution.

3.4.6 Authigenic Transformation of Argillaceous Cement The clay minerals making up the cement of the reservoir rocks on the Saharan Platform are essentially made up of authigenic forms of kaolinite, illite and chlorite. Cementation is porous and of the contact-type and rarely film-like. Our SEM studies have shown that sandstones cemented essentially by epigenetic kaolinite are characterized by a better communication between the pores than sandstones cemented by fibrous illite. This peculiarity becomes even more evident when we compare them to sandstones in which the cement includes modest quantities of illite, detrital chlorite or mixed-layer minerals of the iltite-smectite type which reduce the communication between the intergranular pores even more. The filtration properties of reservoir rocks are thus directly related to the mineralogical composition of the argillaceous cement (Fig. 3.13). However, the authigenic nature of the main clay minerals in the reservoir rocks show that composition, structure and distribution of the sedimentary argillaceous cement have been subjected, especially in highly permeable reservoir rocks, to important postsedimentary transformations. These transformations are controlled by the initial char-

3.4 • Main Controls of the Compaction of Reservoir Rocks

91

acteristics of the reservoir and in turn they themselves exert a great influence on the latter. This is explained to a great extent by the physical and chemical properties of the reservoir rocks, by the dimensions and the arrangement of the clay minerals in the pore space and by the type of cementation experienced by the reservoir rocks. We have noted during our detailed studies of these factors in various sections of the basins on the Saharan Platform that in a number of cases, despite similar or identical values of depths, grain size and the amount of argillaceous cement (in the absence of carbonate cement), the reservoir characteristics can be quite different, with permeability ranging from a few dozen millidarcy to several hundred. Considering that the factors mentioned are here more or less constant the reasons for these differences can only be mineralogy, texture and structure as well as nature of the argillaceous cement. In the cements of the reservoir rocks studied here the altogenic and diagenetic clay minerals (mixedlayer, illite, chlorite and kaolinite) frequently are only a few fractions of microns in size, the particles themselves being isometric, sometimes lamellar elongate but also exhibiting poorly defined shapes with vague outlines. The latter, together with flat grains of other minerals like feldspars, biotite, etc., are arranged in the pore space of the sandy-silty materials mainly in an ordered fashion parallel to the stratification, thereby leading to relatively regular homogenous cementation. It results in an abrupt drop in permeability of the reservoir rocks especially at right angles to the stratification, the permeability in this direction being 5o-65% of that parallel to the bedding. The structure of the allogenic and partly authigenic minerals is generally irregular and the type of cementation may be porous, of the contact-type or mottled. At the same time diagenetic clay minerals appear in the coarse-grained reservoir rocks. They may be distinguished by their relatively perfect shapes and structures, the crystals lying frequently in the pore space in an unordered fashion. The cement may be porous, film-like or of the contact-type. In some cases it leads to an increase of the permeability of the reservoir rocks. The diagenetic kaolinite possesses a rather low sorption capacity (3-5 mg eq-1 per loo g of rock), is of large size (up to lO gm or more) and exhibits individual or aggregated particles with smooth or flat faces arranged in ordered fashion within the pore space of the reservoir rocks (Plate 5). To some extent the kaolinite particles play the same role as the fine-grained silty particles. The replacement of dispersed detrital clay minerals by kaolinite with a higher sorption capacity and an ordered arrangement in the pore space leads to an increase in rock permeability (Fig. 3.13c) even if its proportion is equal to or slightly above that of the allogenic argillaceous cement. An abrupt increase in the content of epigenetic kaolinite in sandstones containing little detrital argillaceous cement (Fig. 3.13c) can, however, lead to a deterioration of the reservoir qualities of the respective rocks. On the whole, the formation of kaolinite at the expense of allogenic argillaceous cement frees part of the pore space and thereby supports the formation of porosity within the cement. This porosity proves to be sufficiently effective in view of the weak absorption capacity of diagenetic perfectly ordered kaolinite. Cements made of anthigenic illite are characterized by a higher arrangement density of the individual particles which do not occur in such large particles as kaolinite and chlorite. It has to be remembered that, on the whole, elongate particles with a micaceous structure frequently containing a certain amount of swelling layers are characterized by a higher dispersion than chlorite and kaolinite. However, the rigidity of the micaceous mineral particles does not lead to a structure of the pore as complicated as in the case of chlorite. As the micaceous species sometimes contain important quan-

9~

Chapter 3 • Main Factors of Reservoir Compaction 30 ~ - ' ' I I, II Oil-(gas-)bearing reservoirs

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, I

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: • %~..~ i & ~ A*." " " , ~a &a• . •~ • 10 100 Permeability (roD)

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Fig. 3 . 1 3 a . Impact of diagenetic mineral cement transformation on reservoir quality, Saharan Basins. A Diagram illustrating the secondary silicification impact on reservoir quality (data selected from Oued el-Mya Basin). B, C Striving for representative results, the studied sandstones were selected so that they have a similar m e d i u m grain size (o.2-o.3 ram), and a close argillaceous cement content (m-~5%) with very little or no carbonate cement

30 ¸

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°.o°o@ ITso (Fig. 3.16). Analysis of quartz from sandstones of the Sahara Plate demonstrates that the ratios of intensity of the doublet bands change with increasing depth and degree of quartz m e t a m o r p h i s m : 18oo >> t78o and Isoo < 178o (Fig. 3.16). The latter reflects the previously unnoticed transformation of the quartz crystalline structure to a less crystalline structure. The nature of the doublet in the 8oo-78o cm -t spectrum of a quartz is due to qs Si-O-Si pulsations of the SiO 4 tetrahedron. Two components (8oo and 78o cm -1) are produced by both the cis- and transgroup of Si20 z. The cristobalite and tridymite spectra are known to include a band in the vicinity of 78o cm -1, which is accounted for by a trans-group. X-ray studies of the structure of quartz, cristobalite and tridymite reveal at least one more type, which is not composed of Si207 ring groups. The structures have the following corresponding bands: 695 (and 392) cm -1 (quartz, cis-isomer), 625 (and 396) cm -1 (cristobalite, transisomer), and 560 (and 380) cm -* (tridymite, cis-isomer). These are the most vulnerable to reduction of crystallinity, and consequently their band intensity varies widely. It is known that cis- and trans-isomers differ in their chemical and physical properties; transformation of trans-isomers into cis-isomers and back requires counteraction of re-bonding. The trans-isomers are usually more stable than cis-isomers, demonstrating a higher energy of x-bonding. Actually, the 790 cm -~ band of crystobalite and tridymite is more stable than the 625 and 56o cm -~ bands. Consequently, it was noticed that with

Fig. 3,16. Infrared spectra of quartz in Triassic, Devonian, Silurian and Cambro-Ordoviclan sandstones from Oued elMya Basin (MGD-1,TEGq, OS-x, GBC-z and OCT-1boreholes). Evolution of quartz doublet 8oo-78o cm-I for sandstones with increasing burial depth: z etalon of quartz; 2 depth 243.55 m; 3 depth 1153m; 4 depth 1558.6o m; 5 depth 1679.3o m; 6 depth 1691.3o m; 7 depth 2 043 m; 8 depth 2978 m; 9 depth 3 o04 m; lo depth 3008.20 m; n depth 3157.8o m; z2 depth 3zn.8o ra; I3 depth 37oo.7o m; ~4 depth 392o.2o In; i5 depth 3996.30 m

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112

Chapter 3 • Main Factors of Reservoir Compaction

3.5 . Silica Solid Phase Transformation: A New Concept for Sandstone Diagenesis

113

~1 Plate 9. Sculpture and morphology of quartz grains at different stages of agglomeration. SEM images of sandstones of the Sahara Plate. a, b (b is a magnified part of a) Individualized quartz grains, well ALR-16, depth z 624.3 m, Middle Devonian (D~). c Faceted quartz grains, well OTLA-1, depth 19zo.o m, Early Devonian (D~). d Closely agglomerated quartz grains, well BH-3oa, depth r398.m m, Middle Ordovician (O0. e Agglomeration of quartz grains, quartzite-like sandstone, well MGD-~,depth 3 955.o m, Cambrian (e). f High density of outlet dislocations on quartz grain cleavage, well GS-1,depth 3r76.zo m, Early Ordovician (Q). g, h Crystal growth steps on quartz grain, well STAH-I7, depth z 9o4.o m, Early Devonian (D1). i Overgrown interstices of quartz grains with Burgers (spiral) dislocation mechanism. The spiral growth of grains is localized around triangular lunules (arrow), well HR-ro, depth 2142.4o m, Late Triassic (T3). j Overgrown junction of quartz grains with Burgers (spiral) dislocation mechanism, well RDC-4, depth 3 799.4 m, Early Ordovician (02) Table 3.4a. Sedimentological and mineralogical characteristics of some studied samples by SEM,shown in Plate 9 (remark: the quantitative mineralogical composition was determined by X-ray diffraction of sample powder and oriented preparation, using mixed multimineral standards; accuracy ±1-2%) SEM Well image

Depth Age (m)

Sedimentological characteristics

Mineralogical composition (%)

Basin

a,b

ALR-16 2624.3 Middle Medium- to coarse-grained, Devon. poorly sorted, rounded with siliceous and clay cement, shallow marine sandstone

Quartz 74;Siderite5; Pyrite 2; Clays 19

Illizi

c

OTLA-1 1920.0 Early Medium-grained, sorted, Devon. rounded, quartzose sandstone

Quartz 79; Siderite 3; Feldspars3; Clays 15

Azzene high

d

BH-301

Quartz 64; Clays 23; Siderite 13

Abner

e

MGD-1 3955.0 Cambr. Medium-to coarse-grained, very Quartz 72;Clays 14; poorly sorted, poorly rounded, Dolomite 8; Feldshallow marine sandstone spars 6

Mouydir

f

GS-S

3 ] 76.2 Early Medium-grained, poorly Ordov. sorted, rounded, very quartzose bar sandstone

Quartz 78; Feldspars 2; Clays 20

Oued eI-Mya

g, h

STAH-17 2 904.0 Early Coarse-to medium-grained, Devon. poorly sorted, rounded, shallow marine sandstone

Quartz 82; Feldspars 3; Clays 15

Ghadames

i

HR-10

2142.4 I_ate Mediumgrained, sorted, argilTriassic laceous, alluvial sandstone

Quartz 73 Feldspars 8; Oued Dolomite 6; Clays 13 eI-Mya

j

RCD4

3 799.4 Early Medium-grained, sorted and Quartz 78; FeldOrdov. well rounded, alluvial sandstone spars8; Clays 14

1 398.1 Middle Fine-to medium-grained, very Ordov, poorly sorted, angular, marine sandstone

Oued eI-Mya

114

Chapter 3 . Main Factors of Reservoir Compaction

3.5 • Silica Solid Phase Transformation: A New Concept for Sandstone Diagenesis

115

Table 3.4b. Sedimentological and mineralogical characteristics of some studied samples by SEM, shown in Plate lO (remark: the quantitative mineralogical composition was determined by X-ray diffraction of sample powder and oriented preparation, using mixed multimineral standards; accuracy +1-2%)

SEM Well image a,b

Depth (m)

Age

Sedimentological characteristics

Mineralogical Basin composition (%)

DL-405

830.6 Middle Devon.

Coarse-to medium-grained, Quartz sorted, rounded, quartzose, Siderite shallow marine sandstone Clays

79 6 15

Illizi

BH-301

1 398.1 Middle Ordov.

Coarse-to medium-grained, Quartz sorted, rounded, quartzose, Clays marine sandstone

84 16

Ahnet

d

HR-10

2149.0

Late Trias.

Medium-grained, sorted, rounded, slighttyshaly, alluvial sandstone

e,f,g

MGD-1 2979.5

Late Trias.

Coarse-to medium-grained, Quartz sorted, rounded, veryquart- Clays zose, deltaic sandstone

92 8

Oued ei-Mya

h

O5-1

2786.9

Early Ordov.

Coarse-to medium-grained, Quartz sorted, rounded, quartzose, Clays bar sandstone

90 10

OuedeIMya

GS-5

3 392.23 Early Ordov.

Medium- to fine-grained, poorly sorted and rounded, shaly, alluvial sandstone

Quartz 86 Feldspars 3 Clays 11

AmguidMessaoud anticlinal system

GS-S

3176.2

Early Ordov.

Medium-grained, poorly sorted and rounded, very quartzose, bar sandstone

Quartz 78 Feldspars 2 Clays 20

AmguidMessaoud antictinal system

k

ME1

2914.0

Cambr.

Medium- to coarse-grained, quartzose, very compacted, alluvial sandstone

Quartz Clays

86 14

tdjeraneM'Zab antictinat system

1

GS-S

3392.0

Early Ordov.

Medium-grained, poorly sorted, rounded, quartzose, bar sandstone

Quartz Siderite Clays

87 3 10

AmguidMessaoud anticlinai system

Quartz 76 Feldspars 7 Dolomite 5 Clays 12

Oued el-Mya

• Plate 10. Evolutional character and aggregation grade for quartz grains with depth and age. SEM images of sandstones of the Sahara Plate. a, b (b is a magnified part of a) Cement has not yet completely filled spaces between grains, they are clearly individual, Area of contact is up to loo microns and uneven, revealing the surface of original grain. In a, grains are bordered by a"coating"-oriented growth of a grain by rhombohedral faces {lo11} (arrow); it shows also large-angle boundaries of grains. Grain size about 3oo-6oo microns.Well DL-4os, depth 830.6 m, Middle Devonian (D2).c Individnalizedgrains, coated by chlorite cement which "prevents" faceting; grain size 15o-3oo microns. Well BH-3m, depth 1398.1 m, Middle Ordovician (02). d Isometricity of faceted grain wirth orientated coating of rhombohedral faces; some grains have a polycrystal coating forming individual portions of future faces (arrow). Size up to 600 microns. Well HRqo, depth 2249.0 m, Late Triassic (T3). e, f, g, h Quartz crystal individuals are clearly identified, with chiefly rhombohedral faces. Spiral depressions are formed by three rhombohedral faces of quartz grains (arrows) arranged after the right-hand or left-hand, 3~ and 3~, screw axis, thus making a fragmem of quasi-closest packing. Photomicrograph g shows a twinning (double arrow) which is closest to the Esterel or the Reihenstein-Grisentallaw. Mean size 15o-3oo microns, e, f, g Wetl MGD-1, depth 2979.5o m, Late Triassic (T3). h Well OSq, depth z786.9 m; Early Ordovician (07). j Quartz monocrystalline individuals with fiat boundaries of convergence, pyramid faces are clearly seen {11zo}. Brazilian-Dauphin4 twinning. Well GS-5, depth 3176.2 m, Early Ordovician (O1). k Complete mergence of grains into a monolithic quartzite-like rock; well-faced quartz crystals (rhombohedral faces) in pores. Well ME-z, depth 2914.o m, Cambrian (C). I Convergence of grains with linear boundaries with spiral growth of grains around triangular lunules (ar~vw).Well GS-5, depth 3392.0 m, Early Ordovician (O~)

116

Chapter 3 • Main Factors of Reservoir Compaction

quartz metamorphism the intensity of 8oo and 390 cm -1 bands of Si20 z cis-groups decreases as the crystalline structure of quartz becomes more compact. Thus, the observed modification of the intensities of 8oo and 78o cm -~ bands of metamorphogenic quartz of Saharan sandstones is due to the increased"contribution" of Si207 trans-groups to vibrations, and can be interpreted as a typomorphic feature of quartz metamorphism. 3.5.4 Discussion 3.5.4.1

Possible Sources of Silica According to existing hypotheses, authigenic quartz can originate from several different sources: precipitation of silica in pores in near-surface conditions (Davis 1964; Blatt 1979), diagenetic transformation of clays and feldspars and other aluminosilicates (Siever 1962; Towe 1962; Keller 1963; Dunoyer de Segonzac et at. 1968; Hower and Eslinger 1973; Hawkins 1978; Blatt 1979), flow of silica-bearing waters from depth (Taylor 195o; Heald 1956), pressure solution (Trurnit 1968; Sibley and Blatt 1976; Robin 1978; Houseknecht 1984) and illitization of montmorillonite (Keller 1963; Dunoyer de Segonzac et al. 1968; Burst 1969; Hower and Eslinger 1973). Silicification of poorly consolidated sediments in near-surface conditions by precipitation from subsurface waters occurs following dissolution of silica and hydrolysis of silicates by surface waters and subsequent reprecipitation in the sedimentation zones by ground water discharge (Thompson 1959; Trurnit 1968). Silicification phenomena can occur under a wide range of geochemical conditions which facilitate dissolution of silicates. The chemical stability of sandstone is a function of temperature and pressure changes during burial; in the Paleozoic sandstones of several basins in the Sahara Plate geothermal gradients are 25 °C km -1. Many of the studied reservoirs are buried to great depth with formation temperatures up to 12o °C and higher. 3.5.4.1.1

Precipitation of Silica from Subsurface Waters We present some dissolved silica data here for waters from various environments in northern Algeria: subsurface waters 5-1o ppm, thermal waters 5-20 ppm, shallow nearshore water 1-2 ppm, fluvial surface water 11-12 ppm. Meteoric waters with lo-15 ppm dissolved silica are known to circulate to depths of many hundred of meters and therefore could present a cement source (Btatt 1979). Although surface water contains some silica in solution which might promote quartz cementation at shallow depth, it is clear that this silica content is too little to account for the amount of authigenic quartz present in Saharan oil-field sandstones at depths of several thousand meters. In the Hassi Messaoud oil field formation water from the Cambro-Ordovician reservoirs contains about 15o ppm dissolved silica, formation water from the Albian reservoirs in the same field contains 15 ppm, while in the Berkaoui oil field Albian formation water contains lO ppm, Triassic water 3.6 ppm and Hassi R'Mel (gascondensate) Triassic formation water contains 5-7 ppm silica. It is obvious therefore that the 15o ppm of dissolved silica found today in Cambro-Ordovician reservoirs could

3.5 • Silica Solid Phase Transformation: A New Concept for Sandstone Diagenesis

127

present a source for sandstone cementation, but the 5-7 ppm of silica in Triassic formation water is not great enough to account for the cementation observed in these sandstones (6-12% of authigenic quartz). Using the same mode of calculation applied by Sibley and Blatt (1976) for Tuscarora orthoquartzites and Blatt (1979) for estimation of the volume of water needed to account for the amount of quartz overgrowths observed shows that some lo 5 to 5 x lo 7 pore volumes of water would have to circulate through these sandstones. This leaves the question unanswered as many Saharan sandstones contain lo-3o% and sometimes up to 35% of quartz cement associated with porosity ranging from 0.5 to 16%, and often with little or no evidence of pressure solution. These sandstones range in age from Cambrian to Triassic, most of them are buried to depths ranging from 1500 to 4 ooo m, and many of them were uplifted after deposition to nearsurface position, a condition necessary to promote quartz precipitation from waters as suggested by Blatt (1979). However, many sandstones uplifted in the Late Paleozoic as a result of Hercynian Orogeny in central and western Sahara which remained elevated in nearsurface conditions up to the present do not show a significant content of authigenic quartz, as was previously demonstrated (Plate 7a). Now, it is obvious that the bulk of quartz overgrowths in Saharan sandstones is not attributable to precipitation from subsurface waters at least because of hydraulic and geochemical considerations. Accepting that dissolved silica in pore waters may in some cases present a source for sandstone cementation by quartz overgrowths, the major source of authigenic quartz is still to be identified. 3.5.4.1.2

Diagenetic Transformation of Clay Minerals: Detritai Iliite Tronsforrnation into Kaolinite The transformation of detrital illite into authigenic kaolinite is observed in all the sandstones studied. This process of illite kaolinization, with silica produced as a byproduct, is widely recognized by geologists (Millot 1964). This process occurs as a result of silica hydrolysis involving a pH reduction, some element migration (Na, Ca, Mg) and immobility of other elements (A1, Fe, Ti...). The quantity of silica liberated reaches 2o% of the dissolved silicate and 15% of potassic alkali is also released. This process may lead to a porosity increase following alkali silicate dissolution. The silica released could be reprecipitated in the absence of a transport medium, thus giving a quartz overgrowth. However, on adjacent grains we can observe quartz grain corrosion resulting from the parallel action of released alkali. These two processes could partially explain the characteristic reservoir heterogeneity in most Saharan basins, but they cannot explain the high authigenic quartz content in the reservoirs studied. In ma W of the sandstones studied quartz cement is up to 17-2o% and the clay content in these rocks only rarely exceeds 15%, but is more usually between 5 and 8%. So if all the detrital clay is illite we derive only 2-2.5% of silica from the total mass transformed. What is more, the iltite transformation into kaolinite is never complete, hence one always observes in the cement a non-transformed detritaI illite, and sometimes detrital kaolinite. A quantitative estimation on the Hassi Messaoud sandstones which contained initially about 3o-5o% of detrital illite (that is, the illite content in the non-transformed siltstones in the same site) shows that only 1% of authigenic SiOa could be derived from the above transformation.

1~8

Chapter 3 - Main

Factors of Reservoir Compaction

3.5.4.1.3

Diagenetic Conversion of Montmorillonite or Interlayered Montmorollonite-Illite to Pure fllite This process may be an important supplier of silica to pore solutions. The free silica released by this mechanism is estimated as z.z g of quartz or chert per lOO g of clay transformed, producing 1.3% of quartz or chert in an ideal case (Towe 1962). 3.5.4.1.4

Diagenetic Alteration of Feldspars and Volcanic Rocks Feldspar dissolution is observed in all the reservoirs studied. Some correlation exists between the occurrence of diagenetically kaotinized feldspars and the amount of quartz overgrowths (Fothergilt 1955; Hawkins 1978), but this phenomenon is generally believed to be quantitatively unimportant as a source of silica (Blatt 1979). Volcanic rock fragments were found to be altered diagenetically in Triassic and some Paleozoic sediments of various basins thus presenting a possible silica source. However, volcanic fragments generally are not produced in the cratonic and shelf environments where most orthoquartzites occur so their diagenetic influence is only local. 3.5.4.1.5

Flow of Silica-Bearing Deep Water by Faulting Tectonics Sandstones with little clay cement are a favourable environment for the circulation of silica-bearing deep waters, especially in provinces with a network of faults (Taylor 195o; Heald 1956). In these conditions some silica could precipitate in the available pore space. In the stud?, area, it was particularly important to examine closely ancient faulting, i.e. Triassic and Cretaceous communicating faults which do not seal between different reservoir blocks. However, studies of the relationship between sandstone permeability and the distance ofboreholes from fractures and faults have demonstrated that the mean permeability of sandstones from boreholes located within 500 m from faults is practically the same as the mean permeability of sandstones from other boreholes. 3.5.4.1.6

Intergranular Pressure Solution A comprehensive petrographic investigation of Sahara sandstones and quartzites has revealed a limited occurrence of intergranular pressure solution even in ancient or deeply subsided sediments. Widespread examination using cathodoluminescence petrography of Cambrian, Ordovician, Devonian, Carboniferous and Triassic sandstones and quartzites from depths ranging from a few hundreds of meters to 5 km has shown that the standard criterion of pressure solution (sutured contacts between apparently detrital parts of adjacent quartz grains) occurs relatively rarely. The great bulk of interpenetrations between adjacent grains occur in fact as compromise boundaries between overgrowths (Plates 6, 7). These overgrowth-overgrowth contacts could not be a source of silica to pore solution (Btatt 1979). The major factors which imply a limited role of pressure solution as source for sandstone cementation in the studied area are: (1) early quartz overgrowths probably at shallow depths, (2) formational overpressure, (3) relatively moderate thermal regime and moderate gravitational stress due to the presence of thick salt formation in the upper part of the stratigraphic column, and (4) cathodoluminescence color of quartz overgrowths.

3.5 • Silica Solid PhaseTransformation: A New Concept for Sandstone Diagenesis

119

In the Sahara sandstones petrographic evidence suggests significant quartz cementation prior to much compaction. Petrographic evidence of early quartz cementation of sands has also been reported by numerous other researchers (Dapples 1959; Siever 1959; Millot et al. 197o; Bucke and Mankin 1971; Blanche and Whitaker 1978). In the Sahara Triassic Province (Oued el-Mya, Triassic, Ghadames and North Illizi Basins), the relationship between quartz overgrowths and evaporites which have penetrated from the overlying Triassic formation into fractured Paleozoic sandstones (salts are deposited in fractures intersecting quartz overgrowths) demonstrates that much quartz cementation in these sandstones (origin other than pressure solution) occurred prior to significant Mesozoic subsidence. Such early significant quartz cement equalizes stress through the rock volume and thus reduces stress concentration at grain boundaries (Sibley and Blatt 1976) and impedes the possibility of pressure solution. So, early quartz overgrowths formed a solid skeleton which resisted subsequent rock consolidation during further subsidence. This phenomenon is one of the factors governing conservation of relatively good reservoir properties at depth in the study area. Saharan basins enclose thick Silurian, Devonian, locally Carboniferous and lesser Triassic shales. These shales were composed initially of a large amount of smectite and interlayered smectite-illite day minerals. Today lo-2o% of smectite-illite in these shales at a formation temperature of 8o-12o °C is quite common. Extensive shale diagenesis, particulary the smectite to illite transformation, produces a water volume equal to approximately one-half the volume of the original smectite according to Powers (1967), as well as the normal expansion of water with increased temperature that accompanies increased burial depth (Magara 1975). This could create overpressured zones in muds and undercompaction in associated sandstones in geoclinal sediments (Bradley 1975; Burst 1976; Hower et al. 1976). In northern and eastern Saharan basins, as well as thick Paleozoic shales, thick Triassic and Jurassic evaporites (up to 2 ooo m) occur universally. The water released during evaporite compaction could create additional stress, leading to formation overpressure and undercompaction which damps, in particular, pressure dissolution. On the other hand, Triassic and Iurassic evaporites provide a regional impermeable seal due to their abundances, uniformity and widespread distribution, conditions required for the conservation of abnormal "excess" fluid pressures. Impermeable seals are also provided by Silurian, Devonian, Carboniferous and Triassic shales as well as by a Triassic volcanic seal overlying Early Triassic or Paleozoic sediments at the level of the Hercynian unconformity. Furthermore, our basin modeling, in particular burial histories, shows that in northern and eastern Saharan basins in the Mesozoic, sediments were rapidly deposited and buried probably more rapidly than the depositional fluids could adjust to the added load. In such situations, the rapidly increasing weight must be partly supported by the fluid that is trapped as a result of the decrease in porosity and permeability during compaction (Blatt 1979). Less favorable conditions for pressure solution occurrences in Saharan sandstones are also indicated by the presence of thick Triassic and Jurassic evaporites in the sedimentary column of the Sahara Plate as a result of their high thermal conductivity and low density compared to clastic sediments. This results in a relatively lower thermal regime and lower geostatic stress. Supporting evidence of a low thermal regime in this province is also derived from quartz cathodoluminescence colors, as was previousely cited.

1~o

Chapter 3 • Main Factors o f Reservoir Compaction

The circumstances above explain why there is little evidence of pressure solution even in Cambrian, Ordovician and other sandstones buried to 5 ooo m and more. The large amount of authigenic quartz viewed in cathodotuminescence microscopy, despite little evidence of pressure solution criteria, suggests that a more appropriate mechanism for silica authigenesis in sandstones needs to be identified.

3.5.4.2 Silica Dissolution-Precipitation A common approach in geology is to analyse processes as a function of temperature and pressure. However, many tong-lived geological processes, in particular the diagenetic processes in sedimentary rocks, develop under moderate temperatures and pressures. As is known, the concept of silica pressure solution in sandstones is shared by many authors (Heald and Renton 1966; Renton et al. 1969; Houseknecht 1984,1988; McBride 1989; Sibley and Blatt 1976; Williams et al. 1985a). Fundamental to supporters of this concept are the laboratory experiments by Fairbairn (1954) in which quartz sand compressively loaded and heated transforms to quartzite. There are several contradictions in the concept of pressure solution. For instance, it is widely believed that stylolites are the result of this mechanism. From a solid state point of view they are most likely associated with the formation of the broken-sinusoidal type of grain boundaries, probably due to their migration. Grain boundaries with a crystallographic disorientation, due to a grain's tendency to regular orientation, can locally bend (Poirier 1983). Silicic acid can get into underground and surface waters as a result of direct dissolution of siliceous rocks and minerals, the weathering of silicates, volcanic activity and microorganisms. The main mechanism suggested for silica extraction from marine waters is the biological activity of diatoms, radiolaria, silicaflagetlates and sponges, and correspondingly the main silica supply occurs in the biogenic zone. The concentration of silicic acid in modern river water is usually within lO-2O ppm, in marine water o.5-3.o ppm and in subsurface water lo-4o to aoo-3oo ppm, depending on the conditions of water saturation (Bogomolov et al. 1967; Schenak and Migovich 1969). The highest content was found in formation waters at depths of 2-3 km, and in hydrothermal waters: from 200-4oo to 5oo-7oo ppm. The content of silicic acid in some sodium carbonate-bicarbonate brines can be as high as 2 70o ppm at pH lo (Jones et al. 1969)- However, natural waters usually show a concentration of silicic acid considerably lower than the solubility limit of amorphous silica under the same conditions (Fournier and Rowe 1962; Table 3-5). Consequently, modern subsurface waters are undersaturated with respect to amorphous silica, while marine water is unsaturated also with respect to quartz (Fournier and Rowe 1977). Some conditions, however, are known which allow mineral crystallization from solution. The main agents of crystallization are the crystallization power and the crystallization rate; crystallization is possible in systems out of equilibrium. The measure of a system's deviation from equilibrium is called the dri~fng force of crystallization, and its actual expression is supersaturation and supercooling. The most important parameter atlowing the growth of crystals from solution is solubility. The concentration of saturated solution quantitatively determines the solubility of the substance under particular conditions. Crystals do not grow from unsaturated solutions, crystals mainly dissolve in them.

3.5 • Silica Solid Phase Transformation: A New Concept for Sandstone Diagenesis

lal

Table 3.5.

Thermodynamic data for mineral forms of silica (all values in k] mo1-1)(O'Connor 1958; Van Lier et al. 196o;Fournier and Rowe1962;Siever1962;Moreyet al. 1967;Walther and Helgensont977) Mineral silica form

Formation Formation Dissolution Solution Energyofacti- Activation enheat freeenergy heat freeenergy v a t i o n o f d i s s o - e r g y o f p h a s e lution in water transformation

Amorphous silica

-898.353

-849.469

cz-Cristobalite -907.510

-853.667

19.2

19.0

c~-Quartz

-858.812

21,7 22.3

21.4 22.9

917.257

] 4.0

15.5

74.5 75 67

25.1

Thus, the crystallization of cristobalite and quartz directly from natural waters seems unlikely. However, the partial precipitation of silica in amorphous form is possible in areas where fluvial and marine waters mix. Precipitation of silica as gels followed by the formation of opal-like silica is possible in practice only in areas of intensive volcanic activity.The heat and standard free energy of solution were calculated from the solubility of quartz in water found analytically to be within the temperature range 25-473 °C (Van Lier et al. 196o). Analogous calculations were made for cristobalite and other silica forms (Fournier and Rowe 1962; Table 3-5)The activation energy of the phase transformations of silica was estimated on the basis of the equations describing the kinetics of phase transformations (Emanuel and Knorre 1984). Similar results were obtained in the laboratory under hydrothermal conditions (Mizutani 1966). The hydroxylation-dehydroxylation of silica occurs during phase transitions.The rate of this process and, correspondingly, the activation energy depends on the activating agents, the most important of them being hydroxyl-ions catalyzing dissolution, polymerization and phase transformations of silica. In anhydrous conditions at temperatures of 45o-6oo °C and a pressure of 4 kilobars the activation energy is 197 k] mole -1. In the presence of lO% H20 it drops to as little as 21 kI mole -1 (Naka et al. 1976). The activation energy necessary for the t r a n s f o r m a t i o n of opal-A --+ opal-CT and opal-CT --~ quartz in anhydrous conditions is 89 k] mole-', while in water vapor the activation energy for the transformation of opal-CT ~ quartz is equal to 13.2 kJ mole-l; OH- and water catalyze the transition. According to numerous supporters of the pressure solution mechanism, the following should occur: SiO, gel --~ opal-A ~ silica solution --~ opal-CT --~ silica solution ~ quartz; as well as: quartz (sand) --~ silica solution --~ quartz (sandstone) -~ silica solution --~ quartzite. This cycle of reactions is unrealistic in the subsurface in practical terms due to the low solubility of the mineral forms of silica in water and, consequently, the low solution saturation, i.e. the failure of mineral forms of silica to achieve the saturation necessary for crystallization. The following well-known fact also contradicts the pressure solution mechanism: at great depths, where permeability- is very low, authigenic quartz overgrowths continue to grow. No breaks of Si-O bonds should occur in the solid phase transition of opal-CT (acristobalite) --~ a-quartz, as there is a martensite type of phase transition occurring through cooperative movement of atoms without any breaking of Si-O bonds (Plyusnina ~986).

i22

Chapter 3 . Main Factors

of ReservoirCompaction

3.5.5 Solid Phase Process

Modern physico-chemistry considers two main solid phase processes to be of great importance to geology: phase transitions and agglomeration. Solid phase transitions take place by interaction between adjacent solid matter particles; they can occur with the participation of gas or liquid phases or with the simultaneous participation of both phases. Solid phase transition can be considered in those cases where the primary phase is amorphous and displays polymorphism, while the crystalline phase is finegrained, disordered and initially deformed. The driving force behind crystallization in amorphous gel-like bodies, considered to be super-cooled liquids, is the redundant free energy. The driving force behind the growth of coarser crystals in a fine-grained phase is the redundant free energy of crystals boundaries. Boundary energy may be considered to be a kind of interphase strain. When grains become coarser strain decreases and the driving force of crystallization weakens. At higher temperatures the processes are sharply accelerated. 3.5.5.1

Solid Phase Transformations

Solid phase transformations can be considered as a set of"chain reactions". During these reactions, an intermediate phase (or a state with an increased reaction activity) is created which activates the adjacent reagent lattice. Polymorphic transitions, particularly in low temperature conditions, are accompanied by the disordering of the crystal structure and the fine dispersion of matter and, more importantly, the formation of an initial metastable amorphous phase. Ostwald (1935) formulated the following rules for physico-chemical systems: (1) in any process, the initially originated state is not a state of highest stability with the least free energy, but rather a state of lowest stability with its free energy close to the initial energy; (2) if there exist, between the initial and the final state, a number of relatively stable intermediate states, these states will follow each other in the decreasing order of free energy (rule of stepwise transitions). Therefore, the driving force of crystallization will change through the various stages of the process (Ostwald's steps) due to changes in the volumetric proportions of phases and shrinkage stresses. Solid phase transformations proceed mainly by diffusion. Various mechanisms have been suggested to explain the process of diffusion, in particular point and line defects and ion exchange. Diffusion most often takes place due to the migration, supersaturation and mergence of vacancies and the formation of pores (Friede11964). As already demonstrated, for relatively recent sedimentary rocks and weathering crusts (20-20o Ma), the characteristic sequence of silica transformation is: silica gel -~ opal-A --~ opal-CT --+ chalcedony -~ quartz. This is a well studied, polymorphous transformation of the solid phase transition type (~z-cristobalite -+ o¢-quartz) (Plyusnina 1983, ~986, 199o). One aspect of the solid phase transformations is the gradual transition of one silica phase to another and the gradual transformation of morphological features down the stratigraphic column with increasing burial depth and age (similarly in weathering crusts). The observed sequential cristobatite-quartz ratio would be unlikely to occur in rocks with repeated dissolution and crystallization from solution (Plyusnina 1983).

3.5 - Silica Solid Phase Transformation: A New Concept for Sandstone Diagenesis

123

3.5.5.2

Agglomeration Sandstone studies have disclosed that in older sedimentary rocks (2oo-25o Ma) another type of solid phase transformation, agglomeration, takes place. This phenomenon can be simulated in the laboratory only at rather high temperatures. However, despite the relatively moderate temperature and pressures characteristic of buried sediments, in our opinion the duration and the specific features of the process of ageing justify the interpretation of these transformations in sandstones as the process of agglomeration. Agglomeration is an irreversible process of transition of an unstable system into a more stable state by the spontaneous consolidation of a dispersed porous body (Tretyakov 1978). This process can be accelerated as well as decelerated by changing burial diagenetic parameters. The final outcome of agglomeration is the formation, from a conglomeration of sand grains weakly bound by forces of adhesion and friction, of a solid monolithic quartzite rock. Essentially, agglomeration is the elimination of the pores in a porous body through their filling with material. Then the"final" density of the rock is reached; resistivity, hardness, chemical stability and thermoconductivity increase, while gas and water permeability decrease. The driving force of the process of agglomeration is surface energy. At the start, the system consists of sand with a large internal interphase surface far from thermodynamic equilibrium and possessing elevated reserves of free surface energy. Each system tends towards reduction of interphase surfaces and this is equivalent to a reduction of the surface energy and, consequently, the total energy of the system. The sandstones of the Sahara Plate illustrate the different stages of agglomeration. In the initial stage, consolidation of a body of grains takes place by the gliding of particles along grain boundaries in response to the great surfaces and excessive free energies that create pressures which tend to compress a body of grains to reduce its free surface. This compressive pressure is given as p ~ ( a / p ) P (1 - Po), where p is surface tension at the boundary of a solid phase, r is grain radius, P is porosity under a given temperature or in a given age interval and Po is original porosity. Therefore the first phase of sand consolidation is related to movement of whole grains. This movement stops when a compact packing of grains is achieved. After this, compaction is possible only through diffusion, not through the movement of grains. In the initial stage of agglomeration, grains are agglomerated to each other, which results in an increase in their contact area and convergence of their centers. During this initial stage grains still exist as separate entities (Plate 9a,b and Fig. 3.17a). In the intermediate and final stage, consolidation takes place and pores which were once in communication shrink and become isolated (Plate 9d,e and Fig. 3.17b,c). 3.5.5.3

Diffusion The main physical process during agglomeration is diffusion, i.e. a mass transfer leading to an equalization of ions and molecule numbers in a system. The strength of diffusion in a given direction, say the X axis, is described quantitatively by Fick's law: f = - D d~N , dx

124

Chapter 3 • Main Factors of Reservoir Compaction b

c

2 d

e

Pore

Pore

Direction of movement - - - - Vacancies - Material particles

Fig. 3.17. Schematic sketch of agglomeration phases: a initial, b intermediate, ¢ final, d Scheme of pore-filling between grains under solid phase agglomeration: comact line of grains shown by double dotted line, grain boundaries before agglomeration shown by dotted line, grain boundaries after agglomeration shown by continuous line; diffusion direction of vacancies and particles are shown by dotted and continuous arrows, respectively, e Scheme of dosed pore filling in grains under solid phase agglomeration. ~, 2, 3 Pore variously sized, subjected to filling in an analogous manner as the intergranular pore space, f Scheme of agglomeration with participation of the liquid phase, involving a capillary pressure and grain consolidation

wherefis the n u m b e r of partides moving across the unit area in one second and d N ! d x its gradient in the X axis direction. The coefficient of diffusion D is expressed in cm 2 s-I. Its temperature dependence is exponential: E

D = Doe RT

,

where D o is the diffusion coefficient at T = ~o and E is the process activation energy. The activation energy in the surface layer, along the grain boundaries a n d in the volume is correlated: Esurf"< Egr. bond.< Evol. a n d Dsurf"> Dgr. bond.> DvoLFrenkel a n d Pines (1945) suggested that the solid state agglomeration is related to viscous flow or creeping into pores, especially d u r i n g heating, due to surface tension in grains possessing some surface bending. During this process the surface free energy decreases. They also showed that this p h e n o m e n o n is due to substance redistrib u t i o n d u r i n g directed v o l u m e a n d surface self-diffusion. Using the w e l l - k n o w n T h o m p s o n formula it was shown that the pressure above the curved surface of a phase

3.5 - Silica Solid Phase Transformation: A New Concept for Sandstone Diagenesis

125

and the vacancies concentration ~ in the crystal body near its surface depend on the curvature of this surface: ~=¢0(1+

2r•

)

or

A=~r-~o=

2°~° ; r-~ 0 ,

where 4o is the equilibrium vacancy concentration near a flat crystal surface, ~r is the vacancy concentration near the curved surface having a curvature radius r, a is the surface tension coefficient, Vo is the volume of vacancies in the crystal, K is the Boltzmann's Constant, T is the absolute temperature and A is the v a c a n c y concentration near a curved surface. The vacancy concentration increases near surfaces with small r, i.e. high curvature, and the vacancy concentration near a convex crystal surface (r is positive) is less than near a concave one (r is negative). The higher the curvature of a surface in the crystalline phase, the higher the concentration of vacancies near this surface, i.e. a gradient of concentration of vacancies exists (Fig. 3-17a)- With time, and in particular with heating of sandstone masses during burial, this gradient tends towards uniformity. Therefore a directed diffusionat flow of vacancies develops from higher to lower concentrations which is equivalent to counter-flow of particles (atoms, ions) or diffusion in the opposite direction. Figure 3.17a,b shows the overgrowth mechanism in the pore space. Grain boundary contacts have a concave surface and small r of curvature. The vacancies move to the grain surfaces and dissipate there (the place of their dissipation can be also the block borders in crystals, dislocations, microfissures and other defects). Particles are diffused towards the crosspiece, enlarging it and filling the pore. The coefficient of surface diffusion being greater than that of the volume diffusion, the atomic flow is mainly directed to the surface of agglomerating grains. Consequently, the preferential transformation of grains at the level of their surfaces is seen. As part of the flow directed to the surface of the crosspiece is taken away from the intergrain contact area, the grains become yet more closer. The process of small pores filling through their integration is more advantageous in terms of reducing the internal interphase surface and, consequently, the surface energy. With time the vacancy oversaturation decreases during agglomeration and the system moves closer to equilibrium. If grains are moistened, a liquid capillary develops and a concave meniscus is formed with a small negative radius of curvature. Capillary pressure raises the liquid in a capillary expelling it into pores and bringing grains together as the sandstone consolidates or "shrinks" (Fig. 3.18c). The time required for this type of agglomeration is directly proportional to the surface tension on the liquid-solid phase interface, and is inversely proportional to the viscosity of the liquid phase and the size of the solid phase particles. The study of these phenomena in sandstones of different origin seems rather promising. Reaction rate constants increase with temperature such that a lo °C temperature increase increases reaction rates (k) by 2-4 times: E

k = Ae

RT

,

where A is the theoretically calculated constant, E is the activation energy, R is the universal gas constant and T is the temperature.

lz6

Chapter 3 • Main Factors of Reservoir Compaction

Sandstone grains with a distorted crystal structure (clearly demonstrated by outcrops of dislocations on grain shears and assessed at 4 x lo 7 cm -~ (Plates 9f, lob) show a much higher coefficient of diffusion and accelerated mass transfer compared to similar processes in perfect crystals.

3.5.5.4 TwinnmgandMechanismofTrans~rmation Aggregates of weakly bonded sand quartz grains constitute an unstable system. The initially disordered mutual orientation of grains later becomes crystallographically ordered with time and burial. Different kinds of deformation contribute to sandstone transformation during diagenesis. Mechanical deformation appears to produce a greater effect initially: it causes change of the mutual position of numerous particles (grains) of the sand, leads to changes in their form and size and contributes to change of interaction forces between them. Twinning is one of the manifestations of orientation regularities in grains. A preliminary condition for this is that the oriented faceting of quartz grains is analogous to the closest packing. However, the most active deformation is plastic deformation as realized by translational sliding. Sliding propagates along certain directions and preferably along closely packed layers of the crystal structure of quartz (plane {lO11}).Consequently, one kind of plastic deformation is twinning, which is particulary characteristic for closely packed crystal structures. The closest-packed planes are planes with minimum surface energy, and for this reason they are most commonly either faceting planes or slide planes. It has been observed that closest-packed layers are typical of intergrown crystals, or twins (Belov 1976). As mentioned above, one dislocation type is the closest-packing defect: twinningplane ABCABCABC

1

ABCABABABC . packingdefect

The packing defect can be considered as a combination of twinning planes. Therefore, deformation-displacement dislocations are due to sliding along crystallographic planes. On completion of deformation the deformed part of the crystal (or a neighbouring grain) may become the mirror reflection of the non-deformed part (or another grain), and thus twinning is achieved. Extraneous particles (distinctly observable in thin sections), overgrown on the "seeding" grains and regenerated fragments are particulary favourable to the formation of strained slide twins. The crystal straining that occurs under near-surface conditions leads to a local twinning of the crystalline quartz structure and to reorieutation of the twins formed with respect to the host crystal (Gordienko et al. 1966; Karyakina 1974). Quartz twins, by analogy with sticking growth twins, develop when grain closing and consequent additional mutual orientation occur under the action of their own electrostatic field. The energy required for the formation of twinning juncture-twinning across the twinning plane is extremely low (Lawson and Nielsen 1959).

3.5 • Silica Solid Phase Transformation: A New Concept for Sandstone Diagenesis

127

3.5.5.5

Role of the Dislocations in Recrystallization The plastic deformation of quartz generates stress and facilitates the accumulation of a significant amount of elastic energy, which is a major drMng force for recrystallization concomitant with agglomeration. Both types of plastic deformation - sliding and twinning - are due to the movement of dislocations, which renders the crystal highly plastic. Moreover, it is precisely the high-density dislocations, visible as triangular depressions in the grain intergrowth planes, that are responsible for twinning (Plates 9f, g,h, loa,b). The mechanism of dislocations formation by plastic deformation of crystals was suggested by Frank and Read (1952). Dislocations are known to be linear defects of various type capable of generating a proper stress field and, consequently, prone to migration under the action of external forces. The resultant effect is the "slipping" of atomic planes - a plastic deformation, i.e. the breaking and remaking of the bonds between the dislocation line and the adjacent atoms. The creeping of boundary dislocations in the direction normal to the slide plane is effected via attachment of the vacancies to or their breaking away from the plane boundary. This process is directly connected with the diffusional mass transfer and plastic deformation and is ultimately associated with the ageing and the instability of sandstone as a system. A specific role should be attributed to screw dislocations, sinbe they facilitate recrystallization (Verma 1953). In the process of compaction, deformational dislocations in sandstone grains move elastically and interact to form stable articulations or nodules (Fig. 3.18). Nodules of screw dislocations are usually extremely strong (Poirier 1983). Dislocations are characterized by excess energy which is made up by the energy of broken or distorted bonds in the dislocation nucleus and the energy of weak stresses around dislocations (elastic stresses). Due to the excess energy in the area of the dislocation nucleus, the substance porsesses an increased chemical activity and thus growth, dissolution and oxidation always originate in defect areas and proceed more intensively along them. An active screw dislocation, once sprung into existence, becomes involved in the spiral crystal growth. As is known, the growth dislocations are virtually indistinguishable from the dislocations produced by strain, and the latter are typical of sandstone grains. As a screw dislocation has gained access to the surface of a crystal, a step with height equal to the perpendicular component of Burgers vector is produced (Plate 9g,h,i). The preferential crystallization in the direction along this step initiates a spiral growth (Mntaftschiev 198o). Consequently, the spiral growth may be envisaged as centered around a certain defect produced by a slight mutual displacement of the atomic planes within a crystal (or crystals), which in fact is screw dislocation. According to theory, the growth of imperfect crystals under weak oversaturation (presumably, one may refer to the diffusional overgrowth of quartz grains) is affected only by screw dislocations (Burton et al. 1951). As mentioned above, spiral pyramids with a screw step (whose height is a multiple of the crystal celt size) are formed. With a step parallel to the closest-packing direction along which the growtl~ rate is minimal, polygonal spirals are formed in such a manner that the face symmetry is retained. It is assumed that the triangular forms (Plates 9i, me,f,h) are most likely associated with partial dislocations which are produced when the atoms become displaced from

128

Chapter 3 , Main Factors o f Reservoir Compaction

d

30

0

1 ,

I0 ,

I00 ~

I000 30

I

I l Oil(gas)-bearing zone I1 III Water-bearing zone

o ~ ' ~

•

o

.o & - . : . .

20

20 •

•

o

oe

mm

~0

• 0

• Oe

•

0° ~ o

•o

o~

,/4Zo

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o o~ %.-,,% ,~ ..o o.o % o O , , 0 o-&..: ~ o • . . "~'%Oo o

.~

o o--~ o. oooo

.

I

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e oooo

o

~

~

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0

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-&-& ~ _ ~ -A4- 4-

4-4d,-

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Ca Ca

_~.

.6-A_4-

N

"0

(~ 20"

: 80 -&

4Y,-ac~-N o

4X-

-6--6-4-

494% u~

o

~

, o

, ~

, o

. . . . . . . m

o

m

o

, tn

o

,

,, , ,

m

o0~r--~

..... m

, ~.

m

, ¢-1

Fe2+/Fe ~+ (%)

Fig. 4.1. Transformation of mixed-layer clays of illite-smectite types (unordered) into ordered phase of vermiculite-chlorite type vs. transition of Fe 2+ into Fe 3+. Paleozoic sediments of the Illizi Basin

134

Chapter 4 . Reservoir D e c o m p a c t i o n and F o r m a t i o n o f A c c u m u l a t i o n C a p a c i t y

short-chained acids become unstable above 2oo °C. The maximum concentration of organic acids is encountered at the same temperature as the ordering of the mixedlayer minerals of the smectic-illite series (Hower 1981). The transformation of disordered into ordered phases of the allevardite type is accompanied by a liberation of iron from the octahedral position in the lattice structure (Hower et al. 1976). Whether the iron is set free and then reduced or whether it is reduced within the structure of a clay mineral to become split off then as ferrous iron (Almon 1974), an electron transfer must have taken place. Oxidation of the organic matter dispersed in the shales and the intense later drop in valence of the iron appear to be the main processes. Chemical analyses of shales from the Saharan basins show moderate to elevated iron contents. It is thus easy to understand why paragenetic minerals enriched with ferrous oxide, like chlorite, siderite and chamosite, formed during diagenesis became so ubiquitous under these conditions.

Fig. 4.2. Schematic generalized distribution of clay minerals on local scale: chlorite local abundance increases towards the sand/shale contact; immediately above is a zone of enhanced porosity; a zone enriched in kaolinite overlies the zone of enhanced porosity

Clay mineral species (%) 0 50 100

~ T 3000 -

u 'ttio n

3100

-

j~~/

Zone of local enhanced porosity

3200 -

O °o ° ° J ~ Zone of anomalous diagenetic chlorite and Fe-rich carbonate

3300 -

=:

:

: :

: • :

lllite Mixed-layer clays Kaolinite Chlorite

4.1 . Decompaction Due to Solution of Binding Compounds of Sandstones

135

Figure 4.z illustrates the typical mineral zoning as a function of depth observed in numerous petroleum-bearing formations of the Saharan Platform. The correlation between porosity and newly formed minerals shows that porosity increases towards the shalesandstone contact. Within the section the content of ferrous authigenic minerals like chlorite, chamosite and siderite is part of the general mineral zoning which also affects minerals such as kaolinite, illite, chlorite and the mixed-layer clays. Such a trend is developed especially where shales act as a source for the iron as well as for the organic solvents. The kaolinitic zone overlying the section represents the limit for the transport of aluminium resulting from the dissolution of the matrix particles of the rock. 4.1.1.1.2

Source of the ]ieids With the increase of temperature during burial the organic matter is subjected to decarboxylation, a process that also takes place in the same way under experimental conditions (Robin and Rouxhet 1978; Rouxhet et al. 198o; Johns 1982). The quantitiy of CO2 produced by the decarboxylation of organic matter in a shale of defined composition depends on the amount and type of the organic matter present. The type kerogen III is the organic matter most prone to the production of COv This type of kerogen which is thermally least mature may contain up to 25% oxygen (Tissot and Welte 1985). According to infrared spectroscopy data (Robin and Rouxhet 1978) the carboxyl groups may contain about 25% oxygen. It is conceivable that an average of 6% of the oxygen in the kerogen may be transformed by decarboxylation into COz. Comparable values were obtained experimentally (Harwood 1977). The amount of secondary porosity formed under the influence of the liberated carbonic acid depends not only on type and concentration of the organic matter but also on the shale/sandstone ratio in the succession considered (Table 4.2). When the dissolution takes place by the above-mentioned directions the amount of CO2 produced by decarboxylation from the organic matter in the shales is large enough to account for a moderate amount of secondary porosity by dissoMng carbonates and feldspars according to the following formulae: CaCO 3 + CO2 + H20

> Ca 2+ + 2HCO;

zKAISi3Oa + zCO2 + 11H20---9 AI2SizOs(OH)4 + 2K++ 4H4SiO4 + 2HCO~ . A balance calculation for different shale/sandstone ratios and types of organic matter shows that in many basins the CO2 formed by decarboxylation of organic matTable

4.2. Secondaryporosity (%) from decarboxylationof kerogen (after Bjorlykke1984)

Type of organic matter

Oxygen

Oxygen in -COOHgroups

Volume of dissolved calcite

Volume of dissolved feldspars 4.4

Ill

25

25

1.6

El

75

13

0.5

1.4

I

I0

7

0.2

0.5

136

Chapter 4 • Reservoir Decompaction and Formation of Accumulation Capacity

ter is not sufficient to explain the entire secondary porosity. In these cases it has to be assumed that the dissolution of the carbonates and feldspars took place under the influence of other organic acids of the acetic and oxalic type.

Other Functional Groups in Kerogens. The oxygen in the organic matter is contained not only in the carboxylic groups but also in other functional groups like ethers, ester complexes, phenols and carbonyl (Rouxhet et al. 198o). From these groups the oxygen is usually split off during higher levels of thermal evolution of the kerogen, probably in the form of COz and H20, although the types of these functional groups during diagenesis are not much known. Inverse Reaction of Leaching. Leaching may take place along the following path with the complementary acid possibly forming during burial (MacKenzie and Garrels 1965): silica + clay + cations

, ~ alumosilicates + water + H +

During the reactions leading to chlorite or an authigenic aluminosilicate, protons in particular will be present in excess because abundant hydroxyl groups form part of the brucite sheets of the chlorite. Numerous successions from the Saharan basins offer clear evidence of a decrease in the chlorite content of the reservoir rocks at the expense of the kaolinite. From this we may suspect that a reaction splitting off the acid may have taken place under the effect of a mineral transformation (Boles and Frank 1979). It should be noted that contrary to commonly held notions the leaching to which the shales are subjected represents rather a trap than a source for these cations.

Mineral Oxidants and Aqueous Pyrolysis Reactions. In the organic matter, concentration and distribution of oxygen in the functional groups clearly limit the amount of C02 produced by simple elimination. Thus, in a certain type of organic matter where the carbon may react with oxygen from an extraneous source, the amount of C02 produced may be much larger than if only the primary oxygen contained in the kerogen were available. In the reducing conditions of deeper horizons it is difficult to establish what had been the source although the oxygen of the H~O could be a potential source. So far it has not become completely clear whether such an aqueous pyrolysis has taken place, although in his experimental maturation of organic matter Hoering (1982) has shown that the exchange of hydrogen between organic matter and H20 is a rather frequent process. Mineral oxidants like ferric iron in the shales have been considered as potential oxidizers of the organic carbon to form CO2 (Eslinger et al. 1979). Again, an extraneous source of oxygen may be available: CH20 + 4Fe3+ + H20

'~

C02 + 4Fe2+ + 4H+

It should be noted that according to this formula 1 mole of protons is produced for every mole of iron reduced. A reaction of this type undoubtedly is of great importance for bringing the materials into solution. The uncertainty, nevertheless, is great as we do not know at a given time how much ferric iron was in the detrital authigenic clays and, consequently, what quantity was available for the reaction with the organic matter.

4.1 . Decompaction Due to Solution of Binding Compounds of Sandstones

~37

Another potential mechanism for the oxidation of kerogen would be the reduction of sulfates or sulfur: SO~- + 2CHaO + H +

> H S - + 2HzO + 2CO a

.

The notable contents of soluble sulfates in the formation waters of the Saharan reservoirs underline the necessity to evaluate the role of this mechanism during the oxidation of kerogens and thus in the increase of the secondary porosity. Role of Meteoric Water. In certain basins the supply of meteoric waters may lead to

the dissolution of cement in the sandstones and to an increase in the secondary porosity. Such a situation appears to be developed in particular in the Illizi Basin where in the outcrop zones the supply of surface waters and their eventual discharge in buried horizons are important phenomena. 4.1.1.1.3

Origin of Organic Species in Reservoir Waters Aliphatic acids, the most important constituents of reservoir waters, are found in the formation waters, organisms and sedimentary rocks in such high quantities that they may probably represent a source of petroleum (Eisma and Jurg 1967; Hunt 1979). The fact that the concentrations of dissolved organic acids in shallow subterraneous waters and formation waters with temperatures below about 80 °C are relatively low shows that the organic species, whatever their origin, and in particular that of the acetates, whether formed by the organisms or from the organic matter in the rocks, have been degraded by bacteria. This degradation of the organic species and their transformation to CO 2 and hydrocarbon gas probably stops at temperatures of about 8o °C (Davis 1967; Carothers and Kharaka 1978). The maximum concentration of acid (organic) anions occurs in the reservoir waters at temperatures above 8o °C (Fig. 4.3, Table 4.1). The most probable source of these acid anions is the thermocatalytic degradation of kerogens containing an excess of aliphatic acids (Eisma and lurg 1967). The quantity of oxygen especially in type III kerogen is high, the atomic O/C ratio being in the order of 3o%, and appears to be sufficient to explain higher concentrations of the acid anions encountered in the study area. High concentrations of acid anions are also encountered in the gas deposits of the Sahara and in those where liquid hydrocarbons, another potential source of organic acids, probably no longer exist. Kharaka et al. (1983) have shown that a defined portion of the natural gas of the sedimentary basins formed by decarboxylation of these acid anions. A certain portion of these acid anions may have acted as a source for liquid hydrocarbons. Oxygenated as well as nitrous and sulfuric compounds have been detected in small quantities throughout the oils and in highest concentrations in the lubrifactants and sedimentary facies (Hunt 1979). Oxygen is present mainly in the resins, asphaltenes and waxes of high molecular mass where its content represents more than 5% of the total mass of the resins. The proportion of heteroatomic heavy compounds usually is below lo% of the oil, but it nevertheless increases with decreasing oil density, reaching values above 5o% in the bituminous sandstones (Tissot and Welte 1978). The anions of the organic acids may be formed through thermocatalytic or bacterial degradation of resins and asphaltenes as in the case of the kerogens.

138

Chapter 4 . Reservoir D e c o m p a c t i o n and F o r m a t i o n o f A c c u m u l a t i o n C a p a c i t y 500 -

500 Zone I

[

e o

o e

Zone 2

Zone3 300

e

300 -

I .o

oN;

!:.

~.% • :V:. -.V " 0%\.

• !'."

o

.1"

"i ..

-a 100

.

it

i

.!o-.: [~'o

t

-. % "." ol

-

"

100 -

i

"

"

"~ 50

50-

~/.o!

Oo/oil

o

.

o

uo

¢p:

ol

FI

20

/o 0 70

50 a

" 20-

I o Triassicreservoirs I • Paleozoicreservoirshighlyuplifted by Hercynianorogeny Paleozoicreservoirsmoderatelyupliftedor non-uplifted I , , I 0 90 110 130 150 170 190 Formation temperature (°C)

Triassic reservoirs

]~ Aliphaticacidanions 3000

.

H2CO-

Paleozoic reservoirs and Triassic reservoirs with immediate contact with Paleozoic (S, D) source shales

r 3 000 I I

3-

~ 2000

~

I- 2000

1000

I000

u

0 80

b

[H, 0

100 120 140 160 80 100 120 140 160 Formation temperature (°C)

Fig. 4.3. Distribution of aliphatic acids and total bicarbonate in oil-field waters, Saharan basins. a Concentrations of aliphatic acid anions (C2-C5) in formation waters from different Saharian basins, in various geological situations. Note that: 1. the highest concentrations are between 80 and lao °C and decrease with increasing temperatures, 2. the lowest concentrations are related to Triassic reservoirs (organic matter-poor shales and low level of kerogen maturation), 3- intermediate concentration are characteristic for paleozoic reservoirs highly uplifted by Hercynian orogeny (involving relatively moderate temperatures and conditions less favourable for organic acides generation), b Average concentrations of C~-C 5 aliphatic acid anions and total bicarbonate in oil field waters for Triassic and Paleozoic reservoirs. All samples with temperatures lower 80 °C have been averaged together; for samples with temperatures higher than 8o °C, averaging is over 20 °C intervals

4.] - Decompaction Due to Solution of Binding Compounds of Sandstones

139

4.1.1.2

Summary of Organic and Mineral Diagenesis The diagrams presented in Fig. 4.4 are a schematic integration of the organic-inorganic diagenesis which may be used for establishing the observed succession of transformations by which porosity increases. The first stage corresponds to eogenesis and immature to semi-mature mesogenesis (2 and 3) prior to the migration of the hydrocarbons. Towards this phase in the upper zone already certain overgrowth rims of quartz are encountered and the sandstones undergo some silicification. The grains of silica with carbonate inclusions for the time being remain unaffected. The appreciable overgrowth rims appear at the end of this stage prior to the deposition of secondary clayey (and mixed-layer) cement. The cement then occupies a certain volume but by no means all of the primary space of the remaining pores. Schematically, a complex kerogen molecule is made up of an aromatic core surrounded by an aliphatic layer, the mono- and bifunctional carboxylic acids as well as the phenols being tied to the peripheral portions of the molecule. As temperature increases with growing burial the bonds with these peripheral functional groups are broken. As the reactions towards ordering of the crystalline structure of the mixedlayers of the smectite-itlite type in the shales (100-110°C) coincide in time, temperature and space with the formation of the highest concentration of organic acids at 8o-12o °C in the brines of the petroleum-bearing formations we may propose a possible mechanism for the origin of the bifunctional organic solvents. It is clear that the mineral oxidants undergo reduction and that the kerogens concurrently are oxidized, which together with the thermal degradation of the kerogens represents an efficient process generating groups of peripheral organic acids. Dissolved in water these acids may be transferred from the source rocks into adjacent reservoir rocks by waters furnished by compaction and transformation of shales. In the second stage (4), corresponding to mature mesogenesis, the previously generated organic solvents have already migrated into the sandstones. In the lower part of this interval the organic bifunctional acids form metallo-organic complexes with the aluminium contained in the clayey cement and the feldspars transporting the products of this reaction upwards above the sandstones. This removal of aluminium from the argillaceous cement (and the mixed-layer minerals) contributes to the formation of secondary porosity. At this stage a defined dissolution of feldspars might have probably taken place in the presence of organic solvents. In the fluid-rock system kaolinite starts to become deposited in the open pore space as the chemical equilibrium changes and the organic aluminium complexes become unstable. As one moves upward in the sandstone succession the abundance of kaolinite increases stratigraphically, this mineral becoming generally more frequent and occurring in greater quantities in the upper part of the section. Thus, the transfer of material resulting from the complexification of aluminium in the lower zone into the upper zone appears to be efficient enough. At the onset of the second stage the kerogen molecule is made up of an aromatic core surrounded by an aliphatic belt. At this stage the peripheral organic acid groups have already been split off and transported away. As temperature increases the bonds between the aliphatic hydrocarbons and the aromatic core are broken and the resulting hydrocarbons migrate into adjacent reservoirs.

14o

Chapter 4 • Reservoir Decompaction and Formation o f Accumulation Capacity Mineral transformation and textural development

Quartz intergranular pressure solution and quartz overgrowths 1

2

Open-packed clean to muddy quartz sand, fresh feldspar and undeformed muscovite, with interstitial illite. (Minor amounts of glauconite, biotite, chlorite, zircon and rutile not shown)

3

iQ

solution 1

Sequences of stages in the development of quartz pressure solution

Q Ca,

A CQ.

Rhombs of calcite, dolomite or siderite attached to and replacing quartz, kaolinite and any remaining feldspar.Authigenic pyrite, mainly cubic

B Rest of pore space filled by large crys~cals of poikilotopic calcite. Further growth of siderite and pyrite, some of which changes to marcasite

D ~

Quartz euhedra grow into pore spaces. Feldsparsmostly destroyed. Kaolinite occupies much pore space.Muscovite frays, forming sericite and kaolinite. (lllite in~reasesin crystallinity

Schematic illustration defining detrital quartz (D.Q.) quartz cement (Q.C.) and overlap quartz (O.Q.)

Different generations of quartz overgrowths

D

e

t

r

e silica

i

t

~

Silica dissolved by pressure solution to give sutured contacts with reduction in pore space.Feldspars partly dissolved. Muscovites bent

~

Schematic diagram of major features of quartz overgrowths

Key Ca =Calcite Ch =Chert D =Dolomite F = Feldspar I = Illite

K =Kaofinite Q =Quartz M = Muscovite Se = Sericite Ma =Marcasite Si = Siderite Po = Pore space Py = Pyrite

Fig. 4.4a. General diagenetic effects within Saharan sandstones: quartz intergranular pressure solution, quartz overgrowths, and mineral transformation and textural development

4.1 - Decompaction Due to Solution of Binding Compounds of Sandstones Mineral authigenesis and pore system evolution

~4~

Kerogenevolution

Non compacted sandrock entirely composedby detrital minerals B

i

t

Mixed-layerclaysgradual conversion.Kaolinitedeve~ lopment. Detrital quartz coating mainly by authigenic chlorite.Slight quartz overgrowths.Some non-ferroancarbonate cement precipitation or replacement.

3

='~ : " q ~ /~7;

)

~ ~ / ,

4 ~

~

/

\

5 ~ , ~ i ~

'

Predominanceof authigenic illite and chlorite as most stable clay minerals. Intensivecarbonate (including ferroan)dissolution. Quartz pressure solution/precipitation. Somepossiblegain of secondaryporosity

Legend Nonferr0an [ ~ - - ~ Mixed-layerclay!~ ~ " Icarbonate Mica ~ Ankerite ~~ Kaolinitedikite I-~-'~--I CNorite [---'--] lllite ~ Feldspar Chert ~ Quartz

Silica Ferroan carbonate Pores

~

!

Furtherconversion ofmixed-layerclaystowardsilliteauthigenesis.Authigenickaoliniteprecipitation,Authigenicchloritedevelopment,Mixed4ayer'sordering corrensite,allevardite.Feldspardissolution.Significant quartzovergrc~c~thsdevelopment,chertdissolution.Carbonatecementdissolution/precipitation.Somesecondaryporosity development. Significant reductionof mixedlayerclaysandtheirfurtherordering~ kalkberg.lmproving x ~ crystallinityofilliteandchlorite. Emergenceofdikite.Feldspar dissolution.Intensivecarbonate cementleashing(especiallydolomite)andpredpitation,aswell asferroancarbonates.Quartz pressuresolutionandothersilicatedissolution.Further(intensive)porosityenhancement.

u

,acidss A--Z

! Organic acids+ liquid hydrcarbons

0

!

Fig. 4.4b. General diagenetic effects within Saharan sandstones: mineral autigenesis and pore system evolution, and kerogen evolution

14a

Chapter 4, Reservoir Decompaction and Formation of Accumulation Capacity

In the third and final stage (5), corresponding to super-mature mesogenesis, after migration of the hydrocarbons only the aromatic core of the complex kerogen molecule will remain. A particularly notable modification of this stage is the deposition of defined quantities of illite and chlorite as well as the intense dissolution of quartz during pressure solution. With continuing burial of the reservoir rocks temperature will increase further and the organic fluids trapped in the reservoirs evoive in situ. The thermal decarboxylation of hydrocarbons results in methane (CH4) as well as in CO2. The thermal degradation of hydrocarbons also forms methane to leave the respective residues in the former contact zone between water and oil as "dead oil". During the same phase the silicate grains together with other components of the cement undergo strong dissolution. The CO~ generated by thermal decarboxylation of organic matter probably increased in many cases the concentration of C02 to a level where its exceeds the concentration in the carbonates. The weak acids present in a system dominated by carbonic acid (CQ) attack the carbonates, creating a higher porosity. In zones without an abundant source of organic fluids there is only a minimal late dissolution of cement. It is obvious that the above-mentioned mechanism is not the only one possible, but its integrated nature makes it attractive. 4.1.1.3

Formation of SecondaryPorosity During Diagenesis Secondary porosity results from chemical, physical, physico-chemical, biochemical and biophysical processes which lead to leaching and compaction of reservoir rocks as well as to the formation of fissures and open cavities. It may take place in the sedimentary shell: (1) under conditions of accumulation of sediments until they undergo greater burial (eogenic porosity), (a) at any depth of burial above the zone of metamorphism (mesogenetic), and (3) during uncovering of the sediments in outcrop after the final phase of burial (telegenetic). Secondary porosity may form in sandstones of any type of mineral composition, textural peculiarity and age. It is encountered especially in sandstones which have undergone relatively long-tasting deep burial and thereby lost their primary porosity. In old sandstones the largest part of the secondary porosity is derived from mesogenetic leaching of carbonates like calcite, siderite and dolomite (Plate 11) as welt as of feldspars (Plate 12) and clays. This decarbonatization eliminates the sedimentary carbonate compounds and the diagenetic carbonates present as cement or replacements of other minerals. A large part of this mesogenetic decarbonatization may be ascribed to the decarboxylation of organic matter during its maturation in shales adjacent to the sandstones, a process that leads to the formation of carbonic minerals and feldspars. Decarbonatization culminates during mature mesogenesis, i.e. at a stage where it notably surpasses carbonization. Because of this, more secondary porosity will form when the primary porosity has been lost. Fissures and non-reducible tamellar porosity evidently assure a sufficient access for the fluids resulting from decarboxylation so that the process of leaching may be initiated even in sandstones of low permeability. An enormous amount of carbonates will be displaced upwards by the solutions away from the diageneticatly mature sandstones to become deposited, at least in parts, in imma-

4.1 . Decompaction Due to Solution of Binding Compounds of Sandstones

143

Plate 11, Carbonate cement in sandstones which suffered intense solution by acids. Note that the carbonate cement in photo c undergoingdissolution is of secondaryauthigenic origin

ture to moderately mature sandstones. In terrigenous sediments undergoing continued burial a large portion of their carbonates will be transformed in a cyclic manner to become displaced further upwards leading to an environment of the sandstones in carbonates at shallower depths. The primary migration of hydrocarbons normally starts when the secondary porosity forms, as during maturation of the organic matter the main phase of hydrocarbon formation takes place just after the culmination of the decarboxylation. This close association of hydrocarbon sources and reservoirs in time and space favours the accumulation of hydrocarbons in the secondary porosity.

144

Chapter 4 • Reservoir Decompaction and Formation of Accumulation Capacity

Plate 12. Siliceouscement (feldspar) undergoing intense dissolution by acids (mesogenetic)

4.1.2 Thermodynamic and Stoichiometric Regime of Formation of Carbonic and Organic Acids and Their Role in the Establishment of Secondary Porosity in Reservoir Rocks In Table 4.3 the calculated pH values for the organic acids for which dissociation constants are available in the literature are compiled. It shows that the concentration of H+-ions which 1 mole of an organic acid can produce ranges from lo -144 mole 1-1 for chloroacetic acid to lo -2.52mole 1-1for trimethylacetic acid. Carbonic acid may furnish

4.1 . Decompaction Due to Solution of Binding Compounds of Sandstones

145

Table 4.3. Dissociation constants and approximate pH values for some organic acids

Acid

Dissociation constant K1

Concentration of H +

Approx. pH

K2

Formic

10 -3.75

I 0 -]88

1,88

Acetic

10-4Js

10 -1.38

2.38

Propionic

10 -487

10 -2,49

2.44

n-Butyric

10 .4.82

10 -2'41

2,41

iso-Butyric

10 .4.82

l 0 -2"41

2.37

Trimethylacetic

10-5.05

10 -2.52

2.52

2.37

Diethylacetic

10

4.75

] 0 ~'37

Chloroacetic

10 -2.87

] 0 -~44

1.44

Glycolic

10 -3.83

] 0-t 92

1.92

Oxalic

10 1.3

] 0 -4.82

l 0 -0"65 - 10 -2"14

0.65 - 2 , 1 9

Carbonic

10 63s

i04o2s

10 4"~6 -- 10-s~

3.2

Table 4.4. Calculated pH values for some organic acids and their Acid dissolution efficiency Carbonic

-5.1

Approx. pH

[H+I

_>3.2

6.3 x 10-4

6x

Efficiency

Propionic

2.44

3.6 x ] 0 4

Acetic

2,38

4.2 x 10-3

7x

Formic

1.88

1.3 x 10 -2

20 x

Oxalic

0.65

2.2 x 10 t

350 x

10-3"16--10 "-Sa moles of H+-ions. Thus, on a stoichiometric basis organic acids may furnish up to three orders of magnitude more H+-ions than carbonic acids. The dissociation constants at z5 °C and the calculated pH values for a number of organic acids either found experimentally by Surdam et al. (1982) or recognized in underground waters by Carothers and Kharaka (1978) are represented in Tables 4.3 and 4.4 which illustrate that the capacity of organic acids to furnish H+-ions is up to 35o times higher than that of carbonic acid. Anions are another factor controlling the stoichiometric efficiency of organic acids. In the case where carbonic acid acts as a solvent for carbonate cement the carbonate ions in solution result from the solvent (acid) as well as from the mineral dissolved. In contrast to this, for acetic acid as the solvent, at least half of the carbonate ions produced during dissolution will be substituted by acetate anions; calcium acetate and not merely calcium carbonate alone is also a product of the dissolution. Calcium acetate is three times more soluble in water than calcium carbonate (Table 4.5). This actually favours the stay of the calcium ions in solution, hindering their precipitation in the form of calcium carbonate. Thus, the higher solubility of the salts of a

~46

Chapter 4 • Reservoir Decompaction and Formation of Accumulation Capacity

Table 4.5. Solubilityof calcium T(°C) Calcium acetate salts corresponding to acids (acetic acid} (in g 1-~) at different temperatures. (after Seidel11965) 0 374.0 10 360.0 20 347.0 25

342.0

30

338.5

Calcium formate (formic acid)

Calcium carbonate

161.5

0.081

-

0.070 0.065

166.0

0.056 0.042

40

332.0

170.0

0.044

60

327.0

t75.0

0.038

80 84

335.0 338.0

179.5

-

85

329.0 311.0

-

-

184.0

-

90 100

297.0

Table 4.6. Comparison of Conversion Gibbs' free energy (AGr°) for carbonic, formic and acetic acids (A1{° represents reduction in the volume of solid phase K-spar > illite products when compared to solid phase reactants) K-spar ~

kaolinite

Anorthite--3 kaolinite

AV~

Converting agent

AG~ (kcal tool-1)

-15.3

H2CO3 HCOOH CH3COOH

+102.6 +95.6 +23.73

-12.5

H2CO3 HCOOH CHsCOOH

+22.98 + 15.8

--1.26 H2CO3 HCOOH CH3COOH

-4.28 -15.0 -165 -36.9

certain acid represents an important factor for intensifyingthe dissociation of the same acid on its own, whereas this is not the case for carbonic acid on its own as suspected by other authors (Lundegard et at. 1984). In Table 4.6, which was compiled on the basis of data on molar volumes and Gibb's free energy values published by Robie et at. (1979), implications for the conversion of certain silicates, for their potential for generating secondary porosity and for a certain thermodynamic regime are compiled. The right-hand column in Table 4.6 presents the reaction energy (AG~') for each conversion series at 25 °C. Comparison of these values shows that AG~) for acetic acid is below that of the other acids, indicating a more advantageous thermodynamic regime. There are two ways that dissolution is possible (Fig. 4.5). The first one is based on the generation of CO2 by decarboxylation of organic matter in the source rocks, the second one on the formation of H + by dissociation of organic acids. In Fig. 4.5 the

4.1 - Decompaction Due to Solution of Binding Compounds of Sandstones

t47

Decarboxylation (carbonic acid hypothesis) ".'.'." zone of :...,.. '-" i:emen1:ati0n,.:

613C = f r o m - 2 t o + 6 % 0

8 ~3c = f r o m - 2 0 t o - 3 0 % o

l

?>i-i i i i.i i i i ,.i.i-i 2Ca2+ + CMO2- + COO2- __>2CaCO3

I )

~i13C= from-2 to -12%0

,Sands?~ane i,i-

iiil;iiiiiil;i;i;iil;ii,iiiill

i i.i.i.;.i,>i-.I.I.;C iiiil;;i;i;iiiiil;iiiiil;il;ill I-/.C i->i-i i.>; i i. . . . . . . . . . . . . . .

2 ) HCMO~ --->H + CMO3-

/

/

HCoO~ -~ H+ + COO2-

CaCMO3+ H2C°O3 --->Ca2++ HCMO~ + HC°O~

(~I3C = from -2 to -8%o

.- .z0ne0f .v... i i.ldissoluiion.}i. -Shale-

I

Dissociation (Organic acid hypothesis)

...'Zon~ o f . . . . dernentati0n : i i i sail;ds}0ne { .i.

O ~-tO

C°O2 + H20 --->H2C°O3 Decarboxylation of organic acids form C002 (50-150 °C)

6~3C : f r o m _ 2

to +6% °

2Ca2+ + CMO2- + 2RCOO- -e CaCMO3+ Ca(RCOO)2 insoluble ~/ soluble

/

H+ + CMO~-

CaCMC03 + H÷ ~ Ca2+ + HCMO~ ' ' .'.

' -.-...' .' •

}i.i-.-z0nei0f}i-i }

. d~sso~ut~on- v -, "-S-6ale- -

RCOOH --~ RCOO- + H÷ (/opt.= 80-120 °C) organic acids

Fig. 4.5. Generation of secondaryporosity due to carbonic or organic acids may be distinguished by the 6~3Cvalues of resulting CaCO3cements, as shown by decarboxylation versus dissociation mechanism. CM carbon of mineral (sedimentary) origin; C0 carbon of organic origin

mechanism for the formation of the acid ions (CO~, H +) in the source rocks, their dissolving influence on the carbonate cement of the adjacent sandstones and the precipitation of the dissolved carbonate in the cementation zone are presented schematically. If decarboxTlation were the dominant mechanism then the (reprecipitated) carbonate cement in the cementation zone should be characterized by a lighter 613C isotopic composition, inherited from the C02 derived from kerogen, compared to that from a carbonate cement of sedimentary mineral origin. If dissociation were the dominant process then it should be controlled by the generation of H + and the carbonate cement would be characterized by an 613C isotope composition similar to that of marine carbonates. Furthermore, ions derived from the organic acids and in particular acetates should be encountered in the formation waters.

148

Chapter 4 • Reservoir Decompaction and Formation of Accumulation Capacity

4.1.2.1

Distribution of Carbon Isotope Composition in Carbonate Cement of the Saharan Reservoirs in Time and Space Studies of calcite concretions collected in shallow sandstones (eogenetic zone) have shown a considerable enrichment in 13C,the said concretions being characterized by fairly low temperatures of precipitation. These values are indicative of sedimentation at a slightly deeper water-sediment interface as a by-product of sulfate reduction (Fig. 4.6).

Fig. 4.6. Stable isotope range (613C) for carbonate cements

-I0

-7.5

-5.0

-2,5

0

+5

I

I

I

I

I

I

+I0

o N

I,.I-I

It" .

I

E K

>_'~

r~

f o Triassic carbonate cement • Paleozoiccarbonate cement • Cabonate associated with sulfate

4.1 . DecompactionDue to Solution of Binding Compoundsof Sandstones

149

In the cement of the Saharan sandstones the residual ferruginous sandstones exhibit a wide spread of fi13Cfrom -lo%o to +6%0, values which agree with those of Irwin et al. (1977). The authigenic ferruginous carbonate cement with heavy fi13C (o to 6%0) apparently was precipitated in the zone of bacterial fermentation (immature mesogenesis) whereas the authigenic ferruginous carbonate cement with lighter ~13C (-5 to -t o%0) rather crystallized in the zone of thermal decarboxytation of organic matter (mature mesogenesis). Cements with fi13Cintermediate between these extremes represent a mixture of the two types. Hmvever, the mineralogical and diagenetic situation is much more complicated than that as the fi13C is not only controlled by diagenetic zoning but also by the type of acid present, viz. carbonic or organic, as outlined above. In the zones of semi-mature and mature mesogenesis we can actually distinguish three fields. The first one is characterized by light fi~3Cvalues (-3 to -12%o) characteristic of well-crystallized carbonate cements in particular in the Triassic reservoirs resulting from decarboxylation and dissolution of the cement by carbonic acid. The second field covers heavy 8~3Cvalues (o to +6%0) in the carbonate cement which is also well-crystallized and developed especially in Paleozoic and Triassic reservoirs lying in immediate or close contact with argillaceous Paleozoic source rocks [Silurian (S), Devonian (D), Carboniferous (C)]. The latter situation obviously corresponds to the generation of (essentially) organic acids and their eventual activities during dissociation. This may be explained in particular by the intercalation of the above-mentioned reservoirs with Paleozoic argillaceous beds rich in rather mature matter. Nevertheless, carbonic acid is also present here and its influence on the resulting fil~C has to be taken into account. A third zone may be delineated between the two above ones, being characterized by intermediate 8~3Cvalues of -2.5 to +2%0. The isotopic composition of the carbonate cement is actually the intereference product of a multitude of factors including diagenetic zoning and type of reagent acid. The respective models explaining the assembly of the geological and geochemical aspects of the hydrocarbon-bearing Saharan provinces will be discussed in detail below. 4.1.3 Model of Differential Dissolution and Redistribution of Carbonate Cement with Compaction/Decompaction of Reservoirs in Space as Based on Carbon Isotope Data 4.1.3.1 Triassic Reservoirs of the Northern Triassic Province

Investigations carried out with the aim of establishing characteristics of the carbon isotope composition of the carbonate cement have shown that the Triassic reservoirs overlying the Lower Paleozoic silty-san@ complex (Cambrian-Ordovician) exhibit the following features: heavy carbon isotopes (fi13C= o to +6%0) are associated with initial carbonate cement; in sandstones with a high solution porosity, the pores of which preserve traces of the carbonate cement, the isotope composition of the latter is also hea W (fi13C = o to +6%0);

15o

Chapter 4 - Reservoir Decompaction and Formation of Accumulation Capacity

low porosity is observed in sandstones with recrystaltized basal cement which is dominant in the upper parts of sand bodies and contains isotopicalty light carbon (813C = -2 to -lo%o). The basal cement results from the crystallization of carbonates transported in solution. A rather interesting distribution of the 613C (and of the carbonate cement type) becomes recognizable when we consider the shale beds separating the Triassic sandy reservoirs (in particular A, B and C) from each other (Fig. 4-7)-The sandstones directly overlying the shale horizons are dominated by residual carbonate cement with clear indications of dissolution, high porosity and heavy 613C. In contrast to this, in sandstones (with low porosity) directly underlying the shale horizons (see Fig. 4.9) recrystallized massive cement with a light isotope composition is dominant. This illustrates that the shale horizons separating the Triassic reservoirs play a double role. Firstly, they are the source of the carbonic acid (from decarboxylation of organic matter), the dissolving action of which becomes clearly evident in the carbonate cement of the sandstones directly (or closely) overlying these beds. Secondly, these horizons represent a barrier holding back the dissolved carbonate which is rising from the lower zones under the influence of acid solutions. Under these conditions the reservoirs become blocked directly below the shale layers. There is a peculiar observation: on a regional basis and throughout the Triassic reservoirs there is an isotypicatly heavy carbon in the carbonate cement of the reservoirs in the lower part of the sandy-argillaceous Triassic succession (Lower Series, ReservoirC and Lower Argillaceous-Arenaceous Triassic), i.e. in the lower part of the sandstone beds closest to the Hercynian unconformit?: Along the latter, probably acid solutions enriched in carbonic acid derived from the decarboxytation of organic matter in source rocks of the Silurian and Devonian located to the northwest, southeast and south of the Triassic Basin and the northern part of the Oued el-Mya Basin have been migrating. It has to be underlined that during the later stages of diagenesis of organic matter the migration of the hydrocarbons followed the same routes, i.e. the Hercynian unconformity, from the argillaceous source rocks of the Silurian and Devonian in the above-mentioned areas towards the Triassic (and also Paleozoic) reservoirs in the Triassic Province including the Hassi Messaoud, Ait Kheir fields, etc. Because of their relative instability only few of the organic acids which could have formed in the same Silurian and Devonian source rocks managed to reach the above-mentioned reservoirs. We have to stress the rote played by the diagenesis of clays and organic matter in the Triassic itself in transformation and redistribution of carbonate cement in its own reservoir rocks. Judging from its degree of maturation the organic matter of the Triassic shales has not yet reached the phase of hydrocarbon generation. Their thermal regime lies thus generally outside the main phase of formation and stability of the organic acids (8o-12o °C). The organic matter of the Triassic shales, however, is just at the stage of decarboxylation where it could have furnished a great amount of carbonic acid which would have participated prominently in the dissolution and reprecipitation of carbonate cement in the sandy reservoir rocks with shale intercalations. Judging from the similarity in the variations of isotope composition and porosity resulting from the dissolution and reprecipitation of the carbonate cement the carbonic acid was produced from the organic matter in the Triassic shales in exactly the same way as in the Silurian and Devonian shales. The difference in the role of the Triassic and the Paleozoic

4.1 • Decompaction Due to Solution of Binding Compounds of Sandstones

151

r~

©

o c~

c~

c~

c~

%

8

o

o

t

~o kl.

152

Chapter 4 • Reservoir Decompaction and Formation of Accumulation Capacity

O

o

N

r~ o E

~r M.

4.1 • Decompaction Due to Solution of Binding Compounds of Sandstones

253

:!1 + +.,'i +' ++"5".++ :+' ~,','~l~:+++!~. :P+++++++~+ .~.~

,% ,Nap atle(]

+~

.++

I ++!I

• ' "+

L L +

+ +

+

+°+J

+ + +

+

+~++:::+:::~

-+:~!

++'+-,'~"~,~,~,,]'~t

:.~]i+H,:

~+ ,,,/+ ++ !, : ++',+,~+ ++' +'t:+'+~s+',+!~++:+::++~+l+',;'~:,-,-+:++.,:+,-?+-++,;~+~+~+k,p

1

154

Chapter 4 • Reservoir Decompaction and Formation of Accumulation Capacity

(S, D) shales lies in the fact that the access of the carbonic acid formed from the organic matter of the Triassic shales to the intercalated reservoir rocks was easier than the penetration of the carbonic acid generated from the organic matter in the Devonian and Silurian successions. These are encountered in relatively elongate reservoir regions in the northern part of the Oued el-Mya Basin and in particular at Hassi R'Mel, Bordj Nili, Ait Kheir, Oued Noumer, DET, DEA, GA, etc. Nevertheless, the quantity of carbonic acid generated in the Silurian and Devonian shales is considerably larger than that formed in the Triassic shales as the former strata are much richer in organic matter. The processes diagenetically transforming the reservoirs by dissolution and redistribution of the carbonate cement have been studied in detail, with the giant gas and condensate field of Hassi R'Mel (HR) as an example, in the light of all geological, and in particular structural, sedimentary and tectonic factors (Figs. 4.8, 4.9). The results are described below. In the upper part of the succession in the HR field, and particularly in reservoir A, there is a notable but inhomogeneous cementation of the sandstones by secondary dolomite and anhydrite. Such a cemen)ation is developed to a lesser degree (in the form of spots) in the upper part of the other two reservoirs (B and C) below A. The two types of cement, viz. dolomite and anhydrite, are different in nature. Isotope studies show that the dolomite cement results from dissolution by acid solutions (H2CO3) and its transfer towards the upper part of each of the three reservoirs directly below the impermeable shale beds separating reservoirs from each other. The transfer of the dolomite cement is more or less progressive in nature. This is confirmed by carbon isotope studies as 613C gradually changes from heavy values in the residual cement of the lower portion of each reservoir towards a lighter value in the upper part, passing through intermediate values in the middle sections (Fig. 4.9). The above process is also observed when we take the three reservoirs as a group. Nevertheless, the described trend for the secondary carbonate cement is more evident in reservoir C, less in B and still less in A. This may be explained by the influence of solutions containing carbonic acid derived from Devonian and Silurian source rocks (shales) to the west, northwest, south and southeast of HR which followed the same routes of migration as the hydrocarbons, i.e. along the Hercynian unconformity towards the HR reservoirs. During their upward penetration in the sandstones the influence of the acid solutions naturally weakened to some extent. Under these conditions one should not forget the local internal sources of HzCO3 represented by the Triassic shale horizons separating the reservoirs from each other or the less important shale zones within each reservoir. In the northern part of the HR field there is a pronounced similarity in the sedimentary characteristics of the three reservoirs, expressed by: • a relatively constant thickness; • a rapid variation in grain size accompanied by a series of conglomerate beds; • rapid pronounced variations in the amount of cement and in particular of clay and carbonate cement strongly affecting the petrophysical properties of the reservoirs. Under these conditions the reservoirs exhibit a great petrophysical heterogeneity. Our research has shown that the factor controlling the petrophysical properties of the reservoirs, like in the two other zones and especially in the central zone, is a superimposed diagenetic dolomitization, in particular in the upper portions of the reservoirs.

4.1 • Decompaction Due to Solution of Binding Compounds o f Sandstones

255

.I i "6

i

i o

!

11111

" "= o-a

rX]

o.I

[ 2 ; 1 '

T

~;-=

i° ~.

o

-io

ii

156

Chapter 4 • Reservoir

Decompactionand Formation of Accumulation Capacity

As we have already pointed out, the secondary carbonatization of the sandstone cement owes its origin to the dissolution of carbonates by carbonic acid in the lower parts of the reservoirs and to their reprecipitation from brines entering the upper parts. The source of the respective H~CO 3 is decarboxylation of dispersed organic matter in the shale layers intercalated within the reservoirs and separating them from each other as well as in the Silurian and Devonian source rocks of neighbouring areas. We also have to evaluate the influence of structural heterogeneity on the diagenetic transformation of the reservoirs in the HR field by dissolution of the carbonate cement. This heterogeneity may be ascribed to faults and their extent as well as to the development of open fissures which at places might become important. One can observe that various faults found in seismic profiles are set out in an ordered network throughout a field. Although the throw of the faults is variable the respective variations stay in a fairly narrow range. The distribution of the faults and of their throw shows that with the exception of the northern tectonically more complicated zone the reservoirs A and C can be isolated from each other only locally. In contrast to this, reservoir B which is normally thinner and exhibits a greater sedimentary heterogeneity may be isolated more frequently, in particular in portions where the throw of the faults is greater. Because of the notable development of faults in the northern part of the field there is a structurally favourabte basis for the penetration of acid solutions, with H2CO3 derived from the shales within the Triassic reservoirs themselves as well as in the Devonian and Silurian shales in neighbouring reservoirs. These very solutions lead to the dissolution and subsequent upward redistribution of carbonates in particular in the reservoirs A and C in which there is a higher secondary porosity at the expense of carbonate cement and silicates (feldspars) becoming dissolved. This is exactly the same process which leads to the vertical diagenetic heterogeneity superimposed over the petrography of the respective sandstones especiallyin zones with a denser development of faults, in particular in the northern part of the field (Figs. 4.7, 4.9)R is this network of fractures developed in particular in the central zone where, furthermore, open fractures dominate over closed ones which assures a good communication between reservoirs and contributes to the preferential exchange of the fluids between them. The establishment of such a process here is, moreover, easier as in this zone the thickness of the shale beds separating the reservoirs from each other is negligible. 4.1.3.2

Triassic Reservoirs of the Ghadames Basin and Other Regions of the Triassic Province In the Ghadames Basin and certain parts of the Oued el-Mya Basin where the Triassic reservoirs rest directly (and unconformably) on Carboniferous, Devonian and Silurian shales the mechanism of dissolving and precipitating carbonate cement in the reservoir rocks (as indicated by the distribution of 613C) appears to be rather complicated (Fig. 4.1o). This situation may be ascribed to two different processes of dissolving carbonate cement. In addition to carbonic acid derived from Silurian, Devonian, Carboniferous and Triassic shales in the areas mentioned, the activity of organic acids was noted which formed in the Paleozoic shales. These acids were not subjected to the tong migration along the Hercynian unconformity. As pointed out in the preceding para-

4.1 - Decompaction Due to Solution of Binding Compounds of Sandstones

~57

~'u

~arc Bou Chaffra [emlet El Bazima

El Hamamit(EHTM)

.,%.%

~,%~e, ~ ~T~i~teLi~li~l~

,j~/ "N "~ \ "k

;econd.,middqe

'~

~ '~ ~elict,littie

~a~

Bir Retmaia (BRT-1)

aem

~%h

dem

181.4 TAG)

Age

Porosity (,%1

~ ~5~

........

=~

c~ O O"

~J~ ~ ~ ~ ~

Argillaceouscarbonate Anhydrite Quartzite Shale Limestone

~ Anhydrite cement ~ Eruptiverock TAGS- Upper ~ shale sandstone T A G I - Lower ]triassic complex

~

;291.~~ludan 298~

~-z ~ +2 ~=o~d~ll ry~ali~d Le~L~li~lo ~

¢

Sandstone SiltstoneMari

6 ~3C(~)

-~

TAC~

~s~.~ T ~ :263~ T~I I ;~ s ~

~

~

qTstaized

~

~

Ip ~

dem ~

,

illD~h

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masses of carbonate and silica and leading to the cyclic transformation of the reservoir rocks under the conditions of permanent burial of the overlying sandstone horizons which are blocked or of low mineralogical maturity. The establishment of good reservoir properties in the Paleozoic strata by leaching after the Hercynian orogeny under the influence of regional epirogenesis may be considered as a special case of reservoir decompaction. Conditions favourable for the formation of good reservoirs are developed in the domes on ancient elevated zones where the top of Paleozoic sandstones and compact quartzites (especially of the CambroOrdovician) has been directly subjected to erosion, mainly of that following the Hercynian phase. The erosion which proceded under epigenetic conditions notably improved the reservoir properties regarding capacity of accumulation and filtration. It is exactly under such conditions that the group of reservoir fields of the EI-AgrebHassi Messaoud chain formed. The optimum depth of erosion of the various fields varies as a function of their primary characteristics and of the thickness of the productive sandstones. The large most productive portion of the Hassi Messaoud rise is tied to an erosive zone of 4o-12o m depth. In the E1-Agreb structure the best production is observed in an erosion zone of 30-70 m depth (Figs. 4.19, 4.20). The analogous conditions favouring the formation

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182

Chapter 4 . Reservoir Decompaction and Formation of Accumulation Capacity

of good Cambrian reservoirs by erosion also exist around the Dahar dome which makes this zone rather promising. In regions with sandstones with good reservoir properties at the base of the Triassic, these horizons form good traps whereas the underlying Cambrian sandstones have become non-productive. This situation is encountered in the Telrhemt dome where the Cambrian deposits around the edge of the structure indicated have suffered an erosion of the same magnitude as those of the Hassi Messaoud. However, despite this erosion the Cambrian sandstones do not harbour any economic hydrocarbon accumulations as these materials were trapped by the Triassic sandstones of the topographically higher Hassi R'Mel structure.

Chapter 5

General Geochemical Features of Generation, Migration and Accumulation of Hydrocarbons

5.1 Geochemical Characterization of Potential Source Rocks, Hydrocarbons and Burial Histories 5.1.1 Silurian Graptolitic Source Clays High geothermal gradients are known to be confined to the zones of an uplifted basement, especially in the Hoggar massif and Ougarta shield. The high gradients tend to increase along the E1-Biod ridge, whereas in the Ghadames Basin, with its thick Mesozoic deposits, the geothermal gradients are comparatively low. The regions around the Hassi Messaoud oil field (HM) exhibit relatively low gradients of 2.2-2.7 °C / loo m, and those around the Hassi R'Met gas-condensate field (HR) somewhat higher gradients, 2.7-3.2 °C / 100 m. Ultimately, the formation temperatures of these two oil pools are roughly the same, since the higher HR temperature gradients are counterbalanced by the deeper location of the HM reservoir. It is clear that a number of other factors are responsible for the type of hydrocarbons trapped in these two oil pools. Therefore, in order to gain a deeper insight into the origin of HR and HM hydrocarbons, one must necessarily account for a possible hydrocarbon sourcing from both the Silurian and Devonian shales. The Silurian organic-rich source rocks, located in the area of the Berkaoui oil pool, and in the northwest and north-east of HR, may be regarded as a potential hydrocarbon source for HR. Carboniferous oil-source rocks entered the oil-window in the Nezla (NL) region north-west of HR. These source rocks may be the source for an oil fringe around this gas-condensate pool. The HR hydrocarbons might be generated in the Silurian source rocks in the west (Berkaoui region) or in the south, and could migrate through the Triassic clastic rocks prior to filling the Cambrian HM reservoir. If so, the HM reservoir structure would be filled with hydrocarbons both by lateral and vertical migration from the adjacent Triassic rocks. However, it is highly likely that the HM hydrocarbons were generated in the Devonian source rocks in the western Ghadames Basin. This petroleum could initially migrate via the Lower Devonian sandstones and then via clastic Triassic rocks. On the whole, although the Silurian source rocks are found at present in the "gas window", they have never been strongly warmed up. This apparent contradiction may plausibly be explained by the uplift and erosion of the Paleozoic sediments during the Hercynian orogeny. In many instances, the present-day subsidence depth of source rocks is smaller than it was before the Hercynian uplift. In the Illizi Basin, in particular in its southern and western regions, the deepest subsidence took place prior to the Hercynian uplift. A slight uplift could not cause a substantial temperature decrease;

184

Chapter 5 • Geochemical Features of Hydrocarbons

consequently, the maturation of organic matter continued, although at a somewhat slower rate. Ultimately, the maturity level rose higher than might have been expected for the present-day temperatures. In the Ghadames Basin, however, an appreciable uplift took place, acting to inhibit somewhat the maturation of organic matter (OM). When, past the Hercynian orogeny, the subsidence started going down to the pre-Hercynian depth, the maturation continued at a greater rate. In reality, this maturation rate growth took place in the beginning of the Cretaceous after a thermal "pause" longer than loo million years. The regional estimate of the average total organic carbon (TOC) takes into account both the level of organic matter maturity (since the areas with overmatured organic matter show low TOC values) and the original depocenters. At present, only some areas with the average TOC in excess of 2% have been observed. These areas are, as a rule, associated with the original depocenters which are not at the present time overmature. Apparently, the organic matter-bearing sediments in the adjacent depocenters have suffered a rapid subsidence, which provided for the preservation of kerogen. In overmature depocenters, the organic matter contents were reduced at the expense of hydrocarbon formation; therefore, their source rocks are not necessarily expected to exhibit high TOC values. The available geochemical data provide evidence that the overmature areas with low TOC were initially highest in TOC. The maturation thus proceeded at the expense of depletion in TOC. Viewed in this aspect, two regions, the northern Sbaa and Ghadames, are of special interest. In the latter basin the original TOC contents were higher than the present-day residual TOC owing to the mature kerogen which was beyond its peak of hydrocarbon generation. This implies that the oil generation potential in this region is higher than might have been expected on the basis of the average TOC values only. This issue will be dealt with in greater detail in the following section concerned with modeling studies. In the north of the Sbaa Basin, the original TOC contents reach 9 % which is markedly higher than the present-day average (about 3%). This area may be promising as a prolific hydrocarbon generator. The other area of interest is the Triassic province. In the Berkaoui region, near the Gellala oil pool (Takhouht area included), a zone with both high initial and presentday TOC values is distinguished; it has been assigned to high-potential Silurian source rocks (Table 5.5) which might be feeders for the known Hassi Messaoud and Hassi R'Mel oil and gas pools. The effective-to-general shale thickness ratio is a factor which determines the ability (or inability) of hydrocarbons to migrate from source rocks. In this context, of definite interest for evaluating the amount of generated hydrocarbons were also the less thick shale beds in the Ghadames, Illizi, Triassic and Reggane Basins. 5.1.2 Devonian Source Shales

The Devonian source shales, when compared with the Silurian, appear to be markedly less mature than the Silurian; they have developed to the stage of a gas-and-condensate window only in certain locations of the area of their occurrence. This situation is most clearly observed in the Triassic Basin, where the Devonian thickness at depths greater than 3 km exceeds 1.o kin. Here the organic matter maturity in the Silurian and Devonian shales is markedly different. Indeed, in certain regions of the Devonian roof,

5.2 • Generation and Directions of Migration

185

the organic matter is immature (Ro = o.4-o.5), whereas in the Silurian roof the organic matter is overmature (R > 1.8). It should be kept in mind, however, that mature OM in the Devonian shales occurs within this range - a point which not should be disregarded in evaluating the gas-and-oil potential and the hydrocarbon migration routes. In turn, the initial and present-day contents of the TOC in the Devonian shales as welt as its occurrence in the basins are quite different from those of the Silurian. Thus, the content of TOC in the Late Devonian shales in the Mouydir, Ghadames and Illizi Basins is very high. Here the TOC values are frequently much greater than those of the respective Silurian rocks, especially in the north of the Ghadames and Mouydir Basins. On the other hand, the TOC contents in the Late Devonian shales tend to decrease westwards across the platform. All these features presumably bear relevance to the alteration in both the direction of spread and the quality of clastic material vis-avis the Silurian. On the whole, the Devonian shales, reaching 2 km in thickness, are of greater occurrence than the Silurian shales. Of special interest are the Ghadames and Illizi Basin areas where the thickness of Late Devonian shales is in excess of 5o0 km. To briefly summarize, the Late (and Middle) Devonian source shales are potentially of commercial interest in the Ghadames, South Timimoune and Reggane, whereas the Silurian source rocks are potentially of commercial interset in the basins of the Triassic Province, North Sbaa and North Timimoune. In these regions, these two source horizons display a similar potential for hydrocarbon generation. 5.2 G e n e r a t i o n and Directions of M i g r a t i o n 5.2.1 Generation in the Silurian Source Rocks Silurian Source Shales in Pre-Hercynian Time. There are two regions in which the Silurian is of interest as regards the migration routes in the pre-Hercynian time. One of these comprises the East Timimoune, Ghadames and Triassic Basins. In those areas, the migration proceeded northwards, to a region north of the Hassi R'Mel and Hassi Messaoud oil and gas pools. The hydrocarbons, generated within this region, could presumably be all degraded owing to the uplift and erosion during the Hercynian orogeny. Another region of interest is the region encompassing the Ghadames and Illizi Basins. The hydrocarbons generated within this region appear to have migrated to the numerous present-day oil pools (the F-6 horizon) of the Early Devonian. The amount of hydrocarbons generated by that time was not great; the essential point was that the hydrocarbons migrated to reach, for the most part, those oil pools. Moreover, the general routes of migration towards those oil pools have not changed much since the preHercynian time. Silurian Source Rocks in the Late Triassic. In the Late Triassic, the Silurian zones o f hydrocarbon generation extended, in all likelihood, over a larger part of the platform and local areas of the Ghadames and Illizi Basins. The migration along the E1-Biod ridge was directed southwards far from the buried zones to more uplifted and exposed zones, along the Ghadames-Illizi Basin boundary, invariably tending towards the resevoirs of the present-day oil pools at the F-6 horizon (Lower Devonian).

186

Chapter 5 • Geochemical Features of Hydrocarbons

Silurian Source Rocks at the Present Day, The migration has not stopped continuing towards the F-6 horizon oil pools such as Tin Foy4 and Tabankort in the Illizi Ba~ sin. The migration of generated petroleum proceeds also in the south of the Tindouf and Reggane Basins. Likewise, in the North Sbaa, E1-Biod and South Timimoune Basins hydrocarbons are generated and migrate toward exposed zones. 5,2.2 Generation in the Devonian Source Rocks Devonian Source Rocks in the Pre-Hercynian Time. In the pre-Hercynian time, the Late Devonian was not in possession of large sources capable of generating hydrocarbons. The hydrocarbons generated in the Timimoune Basin might have migrated in any direction. The hydrocarbons that migrated north and west should have been degraded during the Hercynian uplift, whereas the hydrocarbons that migrated in any other direction might have become entrapped. Devonian Source Rocks in the Late Triassic. By the end of the Triassic, the Late Devonian shales in the Illizi and Ghadames Basins started generating hydrocarbons which are at present confined to the Late Devonian reservoirs. In the west of the platform, the hydrocarbons migrated to both subsided and exposed zones. The former, in all probabilitT, were degraded, while the latter could be entrapped beneath the Triassic sak-bearing horizons. Devonian Source Rocks at the Present Day, The present-day Devonian source rocks do not differ much from those of the Silurian. In the Illizi and Ghadames Basins, the hydrocarbons appear to migrate towards the present-day Devonian oil pools in the south and towards the Triassic pools in the north. In the southern part of the Tindouf and Reggane Basins, petroleum might have been degraded on its exit to exposures. In the North Sbaa Basin, the hydrocarbons migrate in various directions in the HBZ area. This formation had not reached the "oil window" phase by the Late Devonian; nonetheless, it may hold promise owing to the migration from deeper horizons. 5.3 Geochemistry of the Triassic Province 5.3.1 Source Rocks in the East of the Province (Ghadames and Illizi Basins) On the whole, the Carboniferous system in the east of the Triassic Province (Ghadames and Illizi Basins) has low to medium contents of terrestrial gas-trend kerogen (TOC = o.1-1%). Because of the poor enrichment in organic matter and limited aerial spread, this system is believed to be a minor source of gaseous hydrocarbons in the Triassic Basin, despite the occurrence of fairly massive Carboniferous shales in the Ghadames and Illizi Basins. In the Late Carboniferous period, kerogen had not reached the main phase of hydrocarbon generation. The structural and stratigraphic traps confined to these sediments hold promise for hydrocarbons.

5.3 - Geochemistry o f the Triassic Province 2000'

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In the Ghadarnes Basin, there are thick Devonian shales (Fig. 5.1) containing large amounts of thermally mature amorphous organic matter with a high percentage of sapropelic compounds. These shales are believed to be an excellent source of liquid hydrocarbons. Extractive compounds of hydrocarbons and the distribution of liquid hydrocarbons in petroleum suggest that the Devonian facies were prolific producers of petroleum, high in both quality and density. The underlying Devonian and overlying Triassic reservoirs, communicating with the maj or Devonian migration routes, offer a very promising prospect. In the Illizi Basin, the Devonian source rocks are essentially congeneric with those of the Ghadames Basin, but are less massive (Fig. 5.1). In the Triassic Basin, the Devonian shales are depleted of organic matter and hold little promise as potential source rocks.

188

Chapter 5 - Geochemical Features of Hydrocarbons

Fig. 5.2. Isopach map of Silurian shales of the Triassic Province

The Silurian sediments in the western Triassic Basin (Fig. 5.2) contain large amounts of very mature organic matter and are therefore potential sources of hydrocarbons. They feature a high percentage of low-molecular normal paraffins. The Silurian shales in the southern Ghadames Basin and the northern Illizi Basin feature a moderate to good enrichment in organic matter (TOC = 0.5-1.5%). This amorphous organic matter is in the late stage of oil generation. The petroleum of Devonian and Triassic reservoirs immediately adjacent to these shales appear to be of Silurian origin. 5.3.2 Source Rocks in the North of the Province (Oued eI-Mya and Triassic Basins) The present-day overall distribution of the Paleozoic shales (Ordovician, Silurian, Early Devonian) was determined by the sedimentation conditions and by the rate of Hercynian erosion of various units in the region. Over the geological history, the thickest Paleozoic shales were localized in the south, south-west and west of the region. Here the primary thickness was at a maximum, whereas the Hercynian erosion rate was at its minimum. At present, the overall thickness of shale beds is 6oo-7oo m in the south, 280-660 m in the west, and 22o-46o m at the center (see Figs. 5.1, 5.2). The major source rocks in the Oued el-Mya Basin are Silurian and Devonian shales and, to a certain extent, Ordovician shales (Figs. 5.1, 5.2, 5.3). The organic matter of the

5.3. Geochemistry o f the Triassic Province .....-30

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19o

Chapter 5. Geochemical Features of Hydrocarbons

Ordovician shales is sapropelic. The Ordovician source rocks in the southern part of the region were the only to enter the oil window at the end of the Paleozoic, when hydrocarbons were mainly generated in these deposits, At present, the gas-and-condensate generation is possible here. In the rest of the basin, these rocks entered the main phase of oil generation in the Late Cretaceous only. The Silurian and Early Devonian organic matter is typically sapropelic, mixed, and humic (Fig. 5-3). It was intensely generating hydrocarbons in the south of the region in the Pateozoic, and in the areas of the other basins in the Mesozoic. At present, the organic matter may presumably persist either at the end of the oil window, or in the gas window and low-molecular hydrocarbons. The radioactive shales of the Early Silurian are good source rocks in the central and north-eastern areas of the region. The Late Devonian and Carboniferous periods have provided satisfactory source rocks in the south-west and north-east of the region. In the Triassic, ]urassic and Cretaceous columns, of common occurrence are rocks depleted of organic matter and exhibiting a low maturation level (Ro = o.3-o.5%), which resulted in the low yield of the source rocks unrealizable under such conditions. 5.3,3 Characterization of Petroleum in the Eastern Area of the Province

On the whole, all the oils appear to be chemically alike, which is suggestive that they are generated from the same or congeneric source rocks. The distributions of saturated hydrocarbons (Clo) and the pristane/phytane ratio do not feature conspicuous distinctions. However, an analysis of the gasoline C4-C z fraction has revealed certain distinctions among the oils. The following five petroleum types could be distinguished from the compositional analysis of normal, branched and cyclic C7 species: 1. Triassic: Hassi R'Mel, Oued Noumer, A~t Kheir, Ben-Kahla, Berkaoui, Guellala,

Takhoukht (TKT), Dra el-Tamra, Makouda, Hassi Keskassa. 2. Cambrian: Hassi Messaoud, Gassi Touile, Hassi Chorghi, Rhourde Nouss. Devonian: Tamadanet. 3. Devonian: Mereksen, Stah (F4, F6). 4. Devonian: La Recul6e, Tiguentourine, Gazel (F v F4, F6). 5. Triassic: E1-Bourma.

These distinctions bear relevance to the thermal maturity and trapping time; however, their relation to the source rocks is evident and cannot be questioned. More mature oils are confined to the Triassic, Nezla, Gassi Touile, Hassi Chorghi and Rhourde Nouss reservoirs; less mature oils are confined to the Triassic reservoirs El-Bourma and to the Devonian (F3) Stah oil pool. The paleotectonic analysis has revealed two structural groups for the trapped hydrocarbons. On the one hand, there are relatively ancient structures with a good cap rock which were existent in the ~l¥iassic (Rhourde el-Baguel, Messdar, E1-Bourma, Khamadat el-Bourma, Hassi Keskassa); on the other hand, there are recent structures that have formed in the late Lower Cretaceous (North Nezla, Gassi Touite, Hassi Chorghi, Rhourde Nouss). Based on the hydrocarbon ratios, in particular the alkane-to-cyclic ratio in the low-molecular fractions, two genetic

5.3 • Geochemistry o f the Triassic Province

19~

petroleum groups can be distinguished. The first group encompasses the oils of Triassic reservoirs North Nezla, Gassi Touile, Hassi Chorghi and Rhourde Nouss. The second group includes the oils of the Cambrian (Rhourde el-Baguel, Messdar) and Triassic (E1-Bourma, Khamadat el-Bourma, Hassi Keskassa) reservoirs. The oils of the Stah, Mereksen and North Tamadanet deposits are compositionally close to the second group of oils. In this classification, the hydrocarbon ratio reflects the decisive role of some or other of the components of the primary organic matter (steranes, tristerpanes, fatty acids). Thus, the oils of the first genetic group are associated with recent formations, whereas the oils of the second group should be assigned to ancient formations (irrespective of the reservoir age). This fact provides an explanation of the occurrence of various petroleum types in the Triassic formations, that is, the age of the structure, rather than the age of a reservoir, is mainly responsible for the distribution of different genetic petroleum types. This conclusion sheds a new light on the time of the regional hydrocarbon migration from the source rocks. The migration of oils of the first genetic group fell into a period between the Late Triassic and the Early Cretaceous, whereas the oils of the second genetic group became entrapped over the course of a period after the Early Cretaceous.

A study of genetic features of the Devonian petroleum in the north of the Illizi Basin (which merges into the southern flank of the Ghadames Basin) will be of prime importance for the gas-and-oil prospecting of the Ghadames Depression, at present poorly explored. Since the Ghadames Depression and the Illizi Basin constitute a single sedimentary basin in the Paleozoic, the presence or, conversely, the absence of genetic links between the oils of these two formations may prove to be crucial for the discovery of hydrocarbons entrapped in the Devonian formations of the Ghadames Depression. 5.3.4 Petroleum Types and Their Variations in the North of the Province An analysis of distinctive features of the occurrence of oils in the northern area of the platform enables their classification into two types:

1. Paraffinic type (saturated n-alkanes) occurring in the Laghouat, Gassi Touile, Hassi R'Mel and North Nezla oil pools. All these oils are confined to Triassic reservoirs, except for the North Nezla which belongs to an Ordovician reservoir. 2. Naphthene-paraffinic type (saturated cycloalkanes, isoalkanes, more rich in naphthenic and aromatic components than the former type), occurring in the Cambro-Ordovician Hassi Messaoud, E1-Gassi, Rhourde el-Baguel, Silurian Oulouga, Triassic Berkaoul and E1-Bourma oil pools. Such a distribution of oils of these two types reveals no significant criteria - either in geography or in age - for the reservoirs. Besides, geochemical variations have been established within each of the specified types, potentially associated with the depth of source-rock subsidence and the migration pathways. Compositional variations in paraffin have also been observed, presumably associated with the physical segregation during the migration. Certain oil fields (E1-Gassi, Berkaoui) were presumably fed from different source rocks. Variations in C1,C2 and aromatics are recorded even within the same oil pool (for example, Hassi Messaoud). These variations in chemical corn-

192

Chapter 5 . Geochemical Features of Hydrocarbons

position in the north of the platform are indicative of (1) eventual occurrence of different source rocks; (2) eventual catagenesis of different level; (3) eventual dissimilarities in the environmental and provenance conditions (aromatic species are presumed to be associated with marine conditions). In short, the geochemical history of the Saharan basins is quite complicated, and the processes occurring in one region cannot be safely extended to another one. Chemical Evolution of Oil. Two hypotheses of petroleum evolution may be suggested: 1. The primary and secondary migrations were coeval with the petroleum generation in the source rocks. The hydrocarbons were entrapped at the beginning of the oil pool buildup and continued to feed it until this deposit became closed or filled up to a maximum. 2. The petroleum generation started after a good cap had formed; during the course of subsidence, petroleum evolved to cracking stage followed by the gas formation as the submergence proceeded. 5.3.5 Conditions for Hydrocarbon Generation in the North of the Province The theoretical subsidence in various areas over time can be reconstituted with reference to the Hercynian unconformity taken for a zero level by adding successive layers of Mesozoic sedimentary layers. Since the Silurian source rocks (and, occasionally, the Devonian) closely underlie the Hercynian unconformity in the northern Sahara, the subsidence depth relative to the Hercynian surface (the bottom of a Triassic reservoir) is in fact the depth of source rock subsidence for a given area. Two major stages of hydrocarbon generation and accumulation can be defined in accordance with the two generation stages, Paleozoic and Mesozoic. Paleozoic Stage. During the Carboniferous period the hydrocarbons generated in the Ordovician and Silurian in the north of the basin (where the subsidence and high paleotemperature were the most favorable for the organic matter maturation) migrated north and north-east. This process was accelerating over the course of Hercynian orogeny as the north-eastern area of the region suffered uplift and the associated erosion. Consequently; the early hydrocarbon accumulations entrapped in the Paleozoic were destroyed in part (if not completely). Part of these accumulations were destroyed owing to the Hercynian erosion, while the other part, altered to a certain extent, was entrapped in the newly produced structural Ordovician and Silurian traps at the end of the Hercynian cyde. Mesozoic Stage. The Mesozoic stage of hydrocarbon generation and accumulation had its onset during the intense subsidence in the north-eastern area of the region, that is, during the sedimentation of terrigenic Triassic rocks and massive TriassicJurassic salt layers. In the course of subsidence, definite structures formed and evolved, concomitant with a partial or complete oil pool collapse. The hydrocarbons migrated from the destroyed traps to form new oil pools with a Triassic or Jurassic cap. Reservoirs may occur in the Triassic (Berkaoui, Guellala) or Pateozoic (Hassi Messaoud, El-

5.3 . G e o c h e m i s t r y

of the TriassicProvince

~93

Agreb) formations. Such oil pools could have started forming in the beginning of the lurassic, after the salt cap had developed sufficient density and impermeability. The subsequent subsidence of Paleozoic source rocks within the Jurassic and Cretaceous periods stimulated the generation, migration and accumulation of newly-formed hydrocarbons in the north of the region. The Triassic and Paleozoic reservoirs were filled up with the hydrocarbons generated by Paleozoic source shales. Most hydrocarbons of Paleozoic origin were located in the Triassic reservoirs owing to the lateral and, to a lesser extent, vertical migration.

Sumary of the Generation History. Two stages - Paleozoic and Mesozoic - are clearly distinguished over the course of tectonic evolution, with the predominance of subsidence processes. They are separated by the Hercynian orogeny. The area of deepest subsidence, characterized by an increase thickness of Paleozoic sediments, is localized in the south of the Oued el-Mya depression. During the course of the Hercynian orogeny, the north-eastern area of the region, much more elevated, was subjected to severe erosion. Over the Mesozoic period, the north-eastern area of the basin exhibited the highest rate of subsidence owing to the tectonic inversion, which resulted in the formation of a salt-bearing basin of TriassicJurassic age. It is seen therefore that the distinct conditions for the accumulation and maturation of organic matter were associated with the specific geological evolution in the north-east and south-west of the region. By the end of the Late Carboniferous period, when the sedimentation cycle was entering its final stage, the highest paleotemperatures were recorded in the south and south-west of the region, reaching 8o °C at the top and 125 °C at the bottom of the Gedinnian sediments. The lowest temperatures were observed in the north of the region: 80 °C at the Ordovician bottom and 50 °C at the Silurian bottom. By the end of the Cenomanian age, the paleotemperatures rose from loo to 125 °C at the Silurian bottom and from 8o to 11o °C at the Devonian. At the present time, the temperature remains at the same level, except for the north-western and south-western areas where it reaches 13o-15o °C. An analysis of the profiles of present-day temperatures and paleotemperatures in the Paleozoic sediments has indicated favorable conditions for the hydrocarbon generation mostly in the south and south-west of the region. As to the hydrocarbon generation in the central and northern areas, it might have taken place mainly in the Mesozoic. 5.3.6 Conditions for Hydrocarbon Generation in the Eastern Area of the Paleozoic Province

The complex evolution of the Ghadames Depression was, on the whole, a decisive factor for the accumulation conditions, for the organic matter type of the Paleozoic sediments, and the degree of their subsequent conversion, as well as for the eventual migration and accumulation of hydrocarbons. Geological processes of crucial importance are recognized to be the Hercynian orogeny and the erosion of Paleozoic deposits. Another important geological process was the regional subsidence in the Mesozoic, responsible for the formation of hydrocarbon pools. The subsidence occurred in par-

194

Chapter 5 • Geochemical Features o f Hydrocarbons

f

30° r--

-< ! -\ ~s o

~m 1~

Boundary of hiqhly ordered structures (sync'linorium antidinorium ...)

;:l 8oundary of first oraer structures (domes,depressions,..)

-~

Precambrian basement

Fig. 5.4, Studied fields

allel with the sedimentation of massive evaporate layers which served as a cap for more than 5o% of the total depression area. The region, located south of the line drawn from Ain Sefra to Zaouia el-Kahla (north-east to south-west) (Fig. 5.4) is a zone where hydrocarbons were generated during the Paleozoic, where the depth of the source rock subsidence was greater than 2.o km. In cases where the subsidence was 3.5 kin, gaseous hydrocarbons formed. Tissot et al. (1975) believed that the Cretaceous period was the earliest time for generation and migration of petroleum in the Hassi Messaoud and Rhourde et-Baguel oil pools. They have put forward the following arguments: (1) Rhourde el-Baguel was an Early Cretaceous horst, with no clear structuration prior to the faulting; (z) the subsidence curves show the deepest burial to have taken place in the Mesozoic (see Fig. 5-5)Tissot and coauthors clearly underestimated the Paleozoic burial, since it seems to be highly improbable that the Silurian shales were buried a mere 1 km beneath the Devonian and Carboniferous sediments which at present are completely eroded by the Hercynian orogeny. Presumably, there were Devonian and Carboniferous sediments at least 2 km thick in the central, deepest subsided part of the depression in the Haiad area (HAD). The uplift that had occurred by the end of the Middle Devonian seemingly eroded the Late Silurian shales to the Middle Devonian on the flanks of the Ghadames Basin, but left the central part of the basin (that is, the eventual site of oil generation) less affected. The paleotemperature regime of the region is suggestive of the fact that the Silurian shales might have reached the oil generation temperature in the Early Carboniferous. Moreover, the uplift at the center of the basin was minimal, so that the necessary condi-

5.3 • Geochemistry of the Triassic Province 0

S

D

C

195 P

T

J

K

CZ

~

~o "

-0.95 ~_ Q

4

-1.3 "~._ -1.8 L~

5

:g

-590 -5'40 -4'90 --440 -390 -340 -2'90 -2~I0 -1'90 -140

-90

-40

0

Geologicaltime (Ma) Fig. 5.5. Generalizedpattern of the main source rocks,burial, generation and expulsionhistories. Solid lines represent the main variant with erosion, dashed lines the variant free from erosion. Long dashed lines represent a stabilization of source subsidence in some areas beginning in the Late Cretaceous and during the Cenezoic. It stands to reason that achieved maturation level during source subsidence is irreversible and Ro values do not change in the course of uplifting

tions for hydrocarbon generation were not interrupted by the Hercynian movements. Given the present-day overmature state of organic matter at the basin's center, the residual potential after the Mesozoic subsidence is not thought to be significant. However, the Silurian shales in the areas adjacent to Hassi Messaoud persist at a relatively low level of maturity ("oil window" level), since they were submerged to shallower depths.

Geological History of the Region in the Paleozoic. The sediment reduction of Paleozoic formations in the south-east of the region at the beginning of the Paleozoic and the absence of Cambrian sediments in the south of the basin (SED-1 area) may be explained by the fact that the comparatively elevated basement in this region suffered subsidence at a lower rate. The same rate of regional subsidence was typical of the central and north-western parts of the region. As to the overall thickness of the Paleozoic formations of the Messdar area (MDR) and Fort Lalleland (FLD), it was some 3.o km and somewhat greater at the centre of the depression. In the Rhourde Nouss (RN) and Keskassa (KA) areas, the Paleozoic formations also could be significantly thick. Thus, by the end of the Carboniferous the average depth of subsidence was about 3.o km for the Ordovician, 2.8 km for the argillaceous Silurian, 2.5 km for the Early and 2.2-1.5 km for the Middle and Late Devonian (see Fig. 5.5). The Hercynian orogeny entailed a very significant uplift of the western and northern parts of the depression, which resulted in the formation, in the central part of the area, of a depression which was progressively subsiding in the south-east direction to form a saddle in the SEDq area. The Paleozoic sediments were highly eroded in the northern and western areas where the Ordovician, Silurian and Early Devonian deposits were exposed directly beneath the Hercynian unconformity.

I96

Chapter 5 . Geochemical Features of Hydrocarbons

Organic Matter of Paleozoic Rocks. The organic matter suffered a profound alteration as early as in the Paleozoic by the action of high temperature and pressure caused by the regional subsidence. Having assumed the temperature rise to be 1 °C per 34 m, one can see that by the end of the Carboniferous the Ordovician rocks were "heated" up to 113 °C, the Silurian clays to lo7 °C, the Early Devonian to 98 °C, and the Middle and Late Devonian, to 9o and 69 °C. It is natural to assume that by the end of the Carboniferous the organic matter (of sapropelic and humic types) of Ordovician, Silurian and, possibly, Early Devonian was capable of generating liquid and gaseous hydrocarbons which, when entrapped, could form oil pools of industrial interest. Presumably, the eroded Silurian and Devonian sediments were also rich in organic matter, that is, they exhibited a high hydrocarbon potential. The regional Hercynian uplift and the subsequent erosion "inhibited" the hydrocarbon generation and caused a partial destruction of accumulated hydrocarbons. The Paleozoic cycle of hydrocarbon generation and accumulation terminated precisely during these processes.

Mesozoic (Triassic) Province. In the region north of the Mn Sefra-Zou~a el-Kahla (Fig. 5.4), the hydrocarbon was generated mainly during the Mesozoic. In the case of Silurian source rocks, represented mainly by radioactive shales, thickness lines were drawn with reference to the Ordovician roof. For the Devonian source rocks, the thickness lines were drawn with reference to the Early Devonian roof (Siegenian). The Lower Devonian roof appears to be the only reliable datum mark for intra-Devonian correlations. Geological History of the Province During the Mesozoic. During the Triassic and Jurassic, the entire Ghadames Depression was subjected to inverse tectonic movements which caused an intense subsidence in the northern, north-western and western areas, uplifted at the end of the Paleozoic.The salt-bearing basin that had formed in this region extended far beyond the depression boundaries. This fact was reflected in the occurrence of massive evaporites in the Messdar, E1-Kret and Fort Lalleland areas, where the evaporites exceeded 1.o km in thickness, much the same as in the Keskassa area (9oo m) also. Shallowest subsidence was observed in the Rhourde Nouss and Ektaia zones as well as in the south-eastern area of the region, that is, in the zones of deepest Paleozoic subsidence where no evaporites were deposited. The thickness of the Triassic and Jurassic sediments was greatest in the deepest submerged zones of the depression, reaching 1.5-1.9 km. Within the overall subsidence, the E1-Kter, Oued el-Tekh, Ha'iad and Ber Rebaia areas experienced the most intense subsidence. In the Cretaceous, the subsidence in the south-east, west and north-west of the region proceeded at a slower rate, with the resultant formation of the Ghadames Depression in its present-day configuration. Thus, the Ghadames Depression is a superposed tectonic Mesozoic structure whose closure took place during the Cretaceous.

Organic Matter and Hydrocarbons. A t the end of the Mesozoic subsidence, the organic matter, deposited in the Pateozoic, once again became subjected to high temperature and pressure and suffered alteration. In the intensely subsided zone, the Ordovician and Silurian sediments reached a depth of 3.8-5.5 kin, Lower Devonian 3.4-4.7 kin, Middle and Late Devonian 2.8-4.2 km and Carboniferous 2.2-3.5 km. Once again, making use of the present-day geothermal gradient of 34 m / 1 °C, we can determine that by the end of the Mesozoic the Ordovician and Silurian sediments

5.3. Geochemistry of the Triassic Province

197

were heated to 137-187 °C, Early Devonian to 125-148 °C and Carboniferous to 89-128 °C. One will infer therefore that during the Paleozoic cycle, only the Ordovician and, in part, Silurian sediments could have realized their petroleum-generating potential, whereas during the Mesozoic, all Paleozoic sediments, including the deepest subsided Carboniferous sediments, were involved in the hydrocarbon generation process (see Fig. 5-5)-A point to be noted is that the generation of liquid hydrocarbons in the Ordovician, Silurian, Devonian and, in part, Carboniferous sediments was at its completion by the end of the Cretaceous. In the course of regional subsidence and tectonic restructuration of the depression, the newly formed hydrocarbons kept accumulating in Paleozoic traps. Here were also trapped the hydrocarbons that migrated from destructed pools. The severe Hercynian erosion which caused the denudation of Paleozoic reservoirs was subsequently a decisive factor in the migration of Paleozoic fluids to the Triassic reservoirs. The massive evaporite layers and appropriate structural conditions favorably contributed to the buildup of large hydrocarbon accumulations in the Triassic reservoirs. The Mesozoic cycle of hydrocarbon generation was completed by the end of tile Cretaceous. The subsequent geological processes produced no substantial alteration in either tectonics of the region or distribution of the oil pools. For the Silurian source rocks, any Mesozoic trap that was formed in the pre-Aptian period stands a good chance of becoming filled up with oil from these rocks. The traps formed during the Austrian orogeny and closed in a post-Aptian period (during the Late Cretaceous or, possibly, the Tertiary) are probably gas-bearing. Promising traps or privileged structures are those located close to the subsided zones in which the Devonian source rocks have escaped erosion. The eastern province is one of such privileged zones because it remained active during the aleozoic and Mesozoic. By the end of the Cretaceous, the Ghadames Depression started generating gas. 5.3.7 Petroleum to Source Rock Correlations

Numerous analyses of oils from the Ghadames Basin (STAH, MRK, WT) possess isotopically light carbon corresponding to kerogen from marine algae and exhibit 5~3C values between -3o.1 and 3o.6%o. It has been noted that oils from pre-Carboniferous source rocks exhibit slightly heavier carbon isotopes than the corresponding kerogens. It can be suspected that the Silurian shales constitute the source rocks in the Hassi Messaoud area as well as in the central and southern parts of the Ghadames Depression (Wadi el-Teh region) although it cannot be excluded that the oil at Wadi el-Teh (WT-1) was locally derived from the Devonian shales. These shales contain amorphous organic matter in association with pyrites indicative of a reducing sedimentary environment capable of preserving kerogen which eventually will result in petroleum. The oils at WT-1 are too mature to be correlated to biological markers. Tissot et al. (1973) tried to use aromatic steranes as correlation criteria. This method, however, finds little support in the data which, on the other hand, indicate an increase of maturity with age. A classification of the oils into "Cambro-Ordovician", "Upper Silurian-Lower Devonian" and "Upper Devonian-Lower Carboniferous" proposed by some authors actually reflects only the effect of maturation and not any differences in

198

Chapter 5 • Geochemical Features o f Hydrocarbons

source rocks. Although the hypothesis possesses a low probability, it cannot be excluded that the oil at WT-t is a displaced accumulation which is forming today from supermature Silurian kerogen and is preserved in a trap on a structural grid. In order to throw light on the problematic correlation between oils and their source rocks we have investigated oils and bituminoids by mass spectrometry together with gas and liquid-phase chromatography. In the Hassi Messaoud and Rhourde el-Baguel fields the oils in the Cambrian and Ordovician sandstones are isotopically light with fi13C = -29.18 to -29.75%o, typical of marine organic matter largely transformed by microorganisms. The oils are characterized by iCI9/ iCzo = 1.o-1. 4, a generally low concentration of isoprenoids in the saturated hydrocarbon series (Pr/nC~z= o.2-o.3) and a clear preponderance of hydrocarbons with a low molecular mass in the n-alkane series (Table 5.1). The Devonian oils in reservoirs D1-T3 of the Illizi Basin are characterised by a fi13C with a rather narrow spread of-28.1o to -29.85%0 and their pristane/phytane ratio of

Table 5.1. Saharan oil isotope and geochemical features No.

Field

No.ofwell

Age

-~13C(%)Pr/P f

Pr//lC17 K=nC15_lz/nC25_27

I, Triassic Province basins I

Hassi Messaoud

152

E-O

29.75

1,2

0,3

2

Hassi Messaoud

152

E-O

29,51

1.0

0.3

5.5

3

El Gassi

7

E-O

29.90

1.2

0.2

4.8

4

Rhourde ei-Bague[

18

E-O

29.t8

1.6

0,2

5.7

5

Rhourde el-Baguel

25

E-O

29.26

1.4

0.2

5.5

6

Guellala

5

S

29.46

1.5

0.2

4.4

5

7

Guellala

8

Hassi R'Mel

9

4.1

T3

30.46

1.4

0.2

2.8

38

T3

30.47

1.8

0.3

3.0

Rhourde Chouff (condensate)

1

T3

28.52

10

Kef el-Argoub

4

T3

30.80

t.6

0,2

2.8

11

Takhoukht

I

T3

29.34

IA

0.3

3.6

2. llliziBasin 12

Tiguentourine

37

DI (F-6) 28.10

1.9

0.2

5.6

13

Tiguentourine

115

D3 (F-2) 29.30

1.7

0.3

3.8 2.0

14

Edjeleh

29

D2 (F-4) 29.13

1.3

0.9

15

Zarzaitine

65

D2 (F-4) 29.85

1.5

0.3

3.6

16

Mereksen

16

DI (F-6) 29.85

1.8

0.3

3.1

17

Tin Fouye

2

D3 (F-2)

28.91

1.5

0.3

4.5

18

Alrar

5

T3

28.44

-

-

T3

29.95

0.3

3.7

3.

Timimoun Basin

19

Hassi Lato

1

1.7

5.3 • Geochemistry of the Triassic Province

199

2.5 is the highest in the succession studied (Tables 5.2, 5.3, 5.4). This appears to be related to an increase in the proportion of kerogen of terrestrial origin in the total amount of organic matter in the source rock. The isoprenoid content here is also low, as it is in the oils of Cambro-Ordovician reservoirs. The oils from the Triassic reservoirs of fields like Hassi R'Mel (HR), Kef el-Argoub (KG), Takhoukht (TKT) or Hassi Lato (LT) are isotopically similar to the oils of the Lower Paleozoic. An exception are the isotopically relatively hea W condensates of Rhourde Chouff, Tiguentourine and Akrar (Table 5.1) as well as the oils of the Itlizi Basin in general (Table 5.4). The oils of the Paleozoic and the Triassic are exceptionally similar in composition to each other. This chemical homogeneity of the oils is characteristic not only of the vertical succession of the productive complex but also for the various sub-basins of the Triassic Province. The pristane/phytane ratio of 1.3-1.75 is typical and there is a clear dominance of low-molecular hydrocarbons in the n-alkane series with nCl~-vl nC2>~7ranging from 2.0 to 5.7 (Tables 5.2, 5.3, 5.4)- The ratio of pristane to nC v does not exceed o.2-o.3, indicative of sufficiently a high level of oil catagenesis and of the absence of influences of epigenesis on the composition of the oils.

Table $.2a. Pristanelphytane data for oils and bituminoids from Saharan basins (Illizi, Atlas and

Oued el-Mya) Field

Well

Depth (m)

Age

Pristane/ phytane ratio

Oil (0) or bituminoid (B)

Carbonif.

1.63

O

Devon. Devon.

1.44 1.50

O O O

Illizi Zarzaitine

ZR-t 15

523

Tiguentourine

TG-1 t 5 TG-128

1 055 1 022

- 1 053 - 1 039

lama

TAM-1

2082

- 2049

Ordov.

1.53

Amasak

AMA-3

1 998

-2008

Ordov.

1.47

O

Tin Fouye

TF

Devon.

1.65

O

Devon.

1.45

O

Devon.

1.65

O

Devon.

1.39

O

Devon.

1.48

O

El Abed Larache

EAL-12

Edjeleh

DL

1 246

Ohanet

OTN~I 19

Askarene

ASK

Gara

Gara-2

1 937

- 1 939

Devon.

1.48

O

Guelta

GLT

2 686

- 2 697

Devon.

1.47

O

Zarzaitine

ZR

Devon.

1.58

O

Rom

ROM-1

Devon.

1,63

O

Cretac. Eocene

2.53 1.63

O O

2 365

- 1 250 - 2 369

Atlas Oued Gueterine

GKN-1 GKN-1

3 250

- 3 254

Oued et-Mya Hassi Messaoud

MD

Cambr.

1.31

O

Rhourde eI-Baguel

RB

Cacnbr.

1£5

O

Haoud Berkaoui

OKJ

Gassi Touil

GT

3 468

- 3 520

Trias.

1.33

O

Trias,

1.62

O

200

Chapter 5 • Geochemical Features of Hydrocarbons

The isotopic composition of the bulk carbon in the oils is sufficiently uniform and varies only within a narrow range of -28 to -3o%o (Table 5.6). On the whole, the oils are isotopically light which is typical of petroleum genetically related to marine source rocks. For the purpose of a correlation the isotopic composition of bituminoids from Silurian and Devonian shales of a very high hydrocarbon potential has been investigated ('Fables 5.5, 5.6). For the correlation of oils and source rocks we have used the carbon isotope composition of oils and bituminoids within probable source rocks as a diagenetic feature

Table 5.2b. Pristane/phytane data for oils and bituminoids from Saharan basins (Ghadames and

Sbaa) Field

Well

Depth (m)

Age

Pristane/ phytane ratio

Oil (O) or bituminoid (B)

HFR

HFR-1 HFR-1 HFR-1 HFR-I RE-1 ZAR-I ANR-I

2687.30 3 276.05 3321.15 3979.95 3454.00 3991.55 3914.90

Carbonif. Devon. Devon. Devon. Devon. Silur.

1,70 1,70 2.50 2.25 1.60 1.65 1.71

8 B B B B B B

Keskessa

KA-1 bis KA-1 bis KA--1bis ZES-1 REA-1

2 706.10 3 072.70 3 213.40 2 818.80 2773.30

Trias. 5itur. Ordovic. Silur. Silur.

1.20 1.60 1.67 1.69

B B B 8 B

El 8orma

EL8-2 ELB-2 HT8-2 HTB-2

2 394,40 2 404,35 2 434,10 2 69t .90

Trias. Trias. Trias. Silur,

1,09 1.43 0.67 1,43

B B B B

Ghadames

Keskessa

KA-2

Trias.

1.62

0

Wadi Teh

WT-2 HTB-1

2420

-2447

Trias. Trias.

159 1,75

0 0

ELB ELB-9

2 419 -

- 2430

Trias. Trias.

1.50 1.50

0 O

Mereksen

MRK-16

2 77t

- 2 796

Devon,

1.35

0

Stah

STAX

2 719

- 2 726

Devon,

t .36

0

570.0 588.0 674.0 6525

Devon, Devon, Devon. Devon.

2.33 2.36 2.14 1.83

O O O O

995 1 013.25 - 1 025 593.1 614.0 -1 013 - 1 025 592 625 764 793

Devon. Devon, Devon. Devon. Devon, Devon. Devon, Devon.

1,94 1.92 2.18 1.83 2.12 1.80 1.92 1.69

O O O O O O O O

Devon,

1.93

O

El Borma

Sbaa Toat

TOT TOT TOT

ODZ-lbis Hassi Lato

LT-1 LT-1 LT- 1 DECH-I DECH-IW LT-1 DECH-I ODZ-1

Toat

TOT-1

555.0577,75671,0633.2 -

555

57.0

5.3 •

Geochemistry of the Triassic Province

zoi

for the source of the hydrocarbons. Under the influence of biological (fermentational) fractionation and diagenetic elimination of isotopically heavy groups like -COH or OCH 3 or polarization, an isotopic composition is established in the kerogen of the source rock which then is inherited by the oil. Given the direct link between the polarity of the compounds and their isotopic fl-factor, five fractions may be distinguished for petroleum and bituminoids which are in increasing polarity: hexane, hexane-benzene, benzene, benzene-methanol and asphaltene. As a result we have obtained an"isotopic portrait" of the different oils from the Triassic Province, i.e. of the Cambro-Ordovician, Devonian and Triassic (Fig. 5.6, Table 5.6). There are two different shapes of the isotopic curves of isotopically light asphaltenes, which is, as has been established, characteristic of the lithofacies of marine source rocks (Peters et at. ~986). The oil families mentioned above may be separated isotopically into two main groups distinguished clearly by their isotopic curves. In the probable source rocks of

Table

5.3. Average of pristanetphytane ratio for oils from Oued el-Mya fields

Field

Welt

Age

No. of samples analyzed

Hassi Messaoud

MD

Cambr.

11

1,07 - 1.79

1,4

EI-Gassi

GS

Cambr.

3

1.33 - 1.40

1.4 1.7

Pristane/phytane ratio

Zotti

ZT

Cambr,

4

1.36 - 1.82

EJ-Agreb

AR

Cambr.

3

1.l 6 - 2.0

1,5

Average: (21)

1.07 - 2.0

1.5

Nord-East Guellala

GLNE

Silur.

1

1.47

Ouloga

OLG

Silur.

1

1.18

Average: (2)

1.18 - 1.47

1.3

1.54 - 2.92 1.15 - 1.62

2.0 1.5

Haoud Berkaoui

OKP

Trias.

4

Ben Kahla

OKJ

Trias.

3

Org

Org

Trias,

1

GueLlala

GLA

Trias.

3

1.43 - ],75

1.5

Nord-East Guellala

GLNE

Trias.

2

1,27 - 2,27

1,8

N, Goussa

NGS

Trias.

2

1.47 - 1.50

1.485

Takhoukht

TKT

Trias.

I

1.4

Draa-Temra

DRT

Trias,

]

2.6 2.2

1.8

Garet-Ech-Chouf

GEC

Tfias.

1

Kef el-Argoub

KG

Trias.

2

1.33 - 1,94

1.6

Average: (20)

].I 5 - 2,92

1.6

2.1

Hassi R'Mel

KG

Djorf Oued-Noumer Air Kheir

ONR

Trias.

8

2.0 - 2.7

Trias.

2

1.67 - 1.68

1,675

-lrias.

5

1.83 - Z]8

2.0

Trias.

3

1.73 - 2,20

1.95

Average: (18)

1.7

].95

- 2.70

zo2

Chapter 5 - Geochemical Features of Hydrocarbons

the oils examined we have studied the isotopic composition of the various fractions of different polarity in the Devonian and Silurian bituminoids. The first group of oils exhibits exactly the same sharp-peaked curves characteristic of the isotopic composition of the bitmninoid fractions of the Saharan shales which also contain isotopically light asphaltenes. However, the isotopic "portraits" of the bituminoids from the Devonian shales differ sufficiently from those of the Silurian and follow the isotopic image of the oils of the second group which is characterized by a spread of fi13C values similar to that of the five fractions investigated (Fig. 5.6). This group covers oils from, e.g., the Stah

Table 5.4. Carbon isotope composition (-fi13C (%0)) of oils and their fractions in Saharan basins Basin

Field

Well

Age

Fraction

General compos.

Asphaltene

Satur. Atom. Timimoune

Guvette de Sbaa

TOT-1 DECH-1 ODZ-1 bis LT-1 ZR-115

D O D D C

29.5 29.8 29.6 29.4 29.2

29,1 29.1 28.9 28.1 28.6

29.4 29.4 29.2 29.1 28.7

Ghadames

Mereksen

MRK-9 MRK-16 Stab-43

D D D

29.4 29.4 29,5

28.1 28.t 27.9

29.1 29.0 29.2

29.1 29.0

282 28.2 28,4 27.6 28.2 28.1 28,1 27,9

28.6 28,9 28.8 28.0 28.8 28.3 28.7 28.4

Stah Illizi

Askarene

ASK-107 ASK-108

Guelta Amasak EI-Adeb Larach Tiguentourine

GLT-t01 AMA-3 EAL-I 2 TG-115 TO-128

D O D D C

Gara

Tamadanet

Gara-2 TAM-1

O

28.8

28.1

28.7

Ohanet-Nord Ohanet-Sud

OTN-119 OTS-133

D D

28.9 29.0

28.1 28,0

28.6 28.6

Ghadames

Wildgat Rom Gassi Touil Rhourde Baguel

8BK-1 Rom-1 GT-I RB-1

D Pz T

C

29.9 30.2 28.6 28.9

29.3 29.5 28.1 28.6

29# 29.8 28,7 29,5

lllizi

Edjeleh DL Tin FouyeTabankort TFT TFT-I Zarzaitine ZT-I

D D D D

29.1 29.1 29.1 29,0

28,5 28,7 28.1 27.8

29,1 29.4 29.3 29.0

29.1 28.5 29.0 29.0 29.0 28.9

Oued eI-Mya HassiMessaoud Guellali~ EI-Agreb EPGassi

MD GLA AR GS

E S C G

29,9 2937 30.16 30.08

29,4 28,53 29.03 29,18

29,7 28,89 29,21 29.31

29.21 29.31

Timimoune

DECH

D

29.68

29.04

29,07

25,57

5.3 • Geochemistry of the Triassic Province -30

-29

-28

203 -27

BenzeneMethanol

-26

~i13C (%0)

~.

,e ze e

~eXanzne-

Asphaltene

~

b

BenzeneMethanol 3enzene

" D " en v. o. n, J, a~

~

-lexane- ~ ]enzene 613C(%o)

-texane_,30

-29

-28

-27 ,

-26,

-25

Fig. 5.6. Correlation of oils to source-rocks system: Carbon isotope composition for five oil (a) and bituminoid (b) fractions in Saharan fields Table 5.5. Carbon isotope composition (-613C (%0)) for bituminoid fractions from Oued el-Mya oil source rocks Well

Depth (m)

Age

Saturated fraction

Aromatic fraction

Resins

ZTE

3155 3 223

Ordovic. Ordovic.

27.43 27.38

27.15 26.32

27.81 26.35

HBA-1

3 309

Silur.

26,77

27.06

26.67

HBA-1 Nord

3 101 3 240

Silur. Ordovic.

28.89 26.22

27.81 26.24

27,98 2723

SAF-1

2 667 2685 2 815 2450.3 2 539.5 2458.68 2 822

Ordovic. Ordovic. Ordovic. Ordovic. Ordovic. Ordovic. Silur,

30.68 31.11 26.72 30.03 28.45 29.89 30,23

29.27 29,80 27,24 27,28 28,23 29.38 28.15

29.56 29.96 27.60 29,28 27,36 28,81 27.76

EA-1

FDN-1

Chapter 5 - Geochemical Features of Hydrocarbons

204

Table 5.6. Carbon isotope composition (-813C (%0)) for five oil and bituminoid fractions from the Traissic Province (Sahara) ( Z o n e F3 corresponds to Middle-Upper Devonian. T A G I - Trias argilogr4seux infGrieur. T A G S - Trias argilo-gr4seux superieur. S I - s~rie inferieure

Field

Well

Depth/ performation

Stratigraphy

Fraction

System/ epoch

Age

Asphal- Benzene- Benzene Hexane- Hexane tene methanol benzene

Oils Hassi R'mel

HR-38

Triassic

Reserv.C

28.52

27.09

26.50

28.92

29,40

Oued Namous

ONE-8

Triassic

Reserv,B

28.35

26.62

26.06

28.02

28.82

Guellala

GLA-16 3394-3534

Triassic

[~ + Serie 29.09 infer,

27,14

26.68

27.76

28.73

28.68

27.35

27.05

28.56

29.30

29.31

27,48

26.72

28.35

28.90

Hassi KA-lbis Keskessa

2710 -2733

Triassic

Draa Tamra

DRT-3

3 597 - 3 605

Triassic

HassiMessaoud

MD-64

3 350- 3472

Cambrian

29.30

27.03

27.21

28.31

30.04

Rhourde 8ague[

RB-t2

2739 -3010

Cambrian

28.82

27.76

27.46

28.14

28.50

Rhourde Nouss

RN-48

2 726 - 2 730

Triassic

TAGS

28.25

28,02

27,92

28.t8

28,60

Nezla

NZN-7

2527 -2572

Triassic

TAG

28,45

28.21

28,12

28.46

29.12

Hassi Chergui

HC-5

2 461 - 2 478.5

Triassic

TAGI

28.74

28.32

28.20

28.26

28.60

Mereksen

MRK-17

2 771 - 2 776

Devonian ZoneF3

28.87

28,70

28.80

28.86

29.20

Hassi

flTG-13

1533- t 572

Triassic

28.95

28.87

28.88

29.13

29.27

TI

TAGS

Tonareg Stah

STAH-24 2722-2726

Devonian ZoneF~

29.t8

28,94

28,96

28.75

29.00

WadiTeh

WT-t

Triassic

TAG

29.40

29,t2

29.10

29.18

29,85

3724-3731

8ituminoids Rhourde Nouss

RNSE-1 2877.4

Siludan

Gothland 27.83

26,21

25.08

27.90

29.73

Takhoukht

TKT-1

3 842.5

Silurian

Gothland 27.30

26.88

25.27

27.84

28,85

Jejessa

KA-lbi

3072,7

Silurian

Gothiand 28.20

26,67

25.78

28.32

29.16

Wadi Teh

WT-1

3 760.1

Upper Devonian

Strun.-

37.28

27,12

26.90

27.05

28.50

Mereksen

MRK-3

2515.85

Upper devonian

27.54

27,33

26,80

26.77

27.12

Tiguentourine

TG-201

1044.27

Upper De- Strun,vonian Tournis.

28.00

27,16

27.26

28.06

28,06

Tournis. Famen.

5.3 • Geochemistry of the Triassic Province

205

(STAH),Wadi el-Teh (WT), Hassi Tuareg (HTG), Mereksen (MRK), Hassi Chorghi (HC) and Nerzla North (NZN) fields. It is interesting to note that all these fields are enclosed in sandy reservoir rocks intercalated with Devonian shales (D 2 + D3) rich in rather mature organic matter. This is convincing proof of the statement that the above-mentioned deposits were derived indeed from Devonian source rocks. The isotope curve of oils from the gas and condensate field Hassi R'Mel (HR) is rather sharp-peaked (Fig. 5.6) which allows us to connect it to Silurian rocks as the source of its hydrocarbons. In the Hassi Messaoud deposits (MD) and at numerous other sites along the western edge of the Ghadames Depression like, e.g., Rhourde elBaguel (RB) and Rhourde Nouss (RN), the isotope ,portraits" assume an intermediate position in Fig. 5.6 suggesting that they were filled by hydrocarbons derived from Silurian as well as from Devonian source rocks. In addition to the obvious supply from Silurian shales throughout the Triassic Province and the region mentioned, we have every reason to accept the highly likely contribution of Devonian shales of the western part of the Ghadamas Basin to the generation of the hydrocarbons filling, in particular, the numerous deposits in the east and south of the Triassic Basin. The geochemical data on the organic matter-rich shales of the Silurian and Devonian of the Triassic, Ghadames and Illizi Basins exhibit good to even perfect correlations with the oils of these basins. The distribution of Clo-Saturated hydrocarbons, their C4-C7 gasoline composition and the geological distribution of the respective source rocks show clearly that the oils of the northern and central Triassic Basin are genetically connected essentially to the black shales of the Silurian. The oils from the south of the Triassic Basin as well as from the south (and west) of the Ghadames and Illizi Basins on the whole are derived from a Devonian source, The oils from the deposits Hassi R'Mel (HR), Makouda (MK), Mt Kheir (AT), Oued Noumer (ONR), Djorf (D]), Dras Temra (DRT), Guellala (GLA), Takhoukht (TKT), Berkaoui (OKP) and Ben Kahla (OK]) were formed mainly in the Silurian organic matter-rich black shales of the central part of the ~l¥iassic Basin. Their migration probably took place along the Hercynian unconformity, filling on the way reservoir rocks of the Triassic as well as of the Cambrian. The lower structural zone separating the deposits of Rhourde el-Baguel (RB) and Messdar (MDR) from those farther west suggests that otis of these two deposits probably originated in Silurian and Devonian shales of the western Ghadames Basin. Furthermore, isotopic and chemical similarities show that the Silurian shales of the Ghadames Basin could also have been the main source of the hydrocarbons of Hassi Keskessa (KA), E1-Bourma (ELB) and of reservoir F-3 of the Stah deposit. The Devonian shales of Ghadames and Illizi which are rich in organic matter have furnished a food part of the hydrocarbons of these deposits. The Devonian shales of the Ghadames Basin generated much petroleum which could migrate westward to fill the Triassic reservoir rocks at Nerzla (NZN), Gassi Touil (GT), Hassi Chorghi (HC), Rhourde Nouss (RN) and Arzel. The oils of the deposits at Tiguentourine (TG), La Recul~e (RCL), Gazel, Mereksen (MRK), Tamadanet (TAM) and Stah (STAH; with the exception of reservoir F-3) could have been derived mainly from the Devonian rocks of the Illizi and the southern Ghadames Basins. Silurian source rocks underlying this region could also have contributed part of the oils in these fields.

Chapter 6

Burial History and Kinetic Modeling for Hydrocarbon Generation

6.1

The Models Numerous computer programs, such as MATOIL, GENEX, TEMISPACK, PDI, and others, have been widely used to reconstruct the burial and thermal histories of source rocks in order to estimate hydrocarbon yields (phase and amounts), identify abnormal pressure zones, predict the reservoir rock properties, and identify oil migration pathways (Welte and Yukler 1981; Galushkin et al. 1985; Berthold and Galushkin 1986; Doligez et al. 1986; Nakayama and Lerche ~987; Tissot et al. I987; Espitali4 et al. 1988; Welte and Yalcin 1988; Ungerer 199o; Ungerer et al. 199o; Lopatin et al. 1992; Galushkin and Kutas 1995; Makhous et al. 1995). The underlying algorithms on which the software packages are based have been described in numerous papers. Because the diversity and complexity of processes involved in basin evolution are extremely specific, software packages created at different times by various researchers are quite different. Furthermore, the results of the numerical analysis are dependent on the basic principles of the model. The purpose of this chapter is to outline the basic principles of the integrated galo thermal modeling program. The main goal of this part of the study has been to present the basic design of the integrated galo software package. In our thermal analysis, the sedimentary blanket, the lithosphere, and the upper part of the asthenosphere are considered together. This approach allows us to calculate the amplitude of tectonic subsidence by considering changes in the density distribution vs. depth in the lithosphere. Local isostatic response of the lithosphere on load is assumed, and then the comparison of relative variations in the amplitude of tectonic subsidence is calculated by traditional methods (removal of the water and sediment load on the basement surface), with the variations obtained by nontraditional methods (consideration of changes in the density profile in the basement) providing an additional opportunity to control the program's sequence of the tectonic and thermal events in the lithosphere. Incorporating the burial stage of the source rock sample in the fitting procedure of kinetic parameters of reactions controlling the organic matter maturation is another key feature of our program package for basin modeling. Consequentl); we can determine the energy spectrum of kinetic reactions for samples with relatively moderate levels of organic matter maturation (vitrinite reflectance Ro = 0.5-0.8%).

6.1.1 The Program Our modeling program consists of three main blocks: input data for basin structure and evolution, initial parameters for basin modeling, and numerical simulation

zo8

Chapter 6 • Burial History and Kinetic Modeling for Hydrocarbon Generation

Input data for basin structure and evolution ] Present-day sedimentary section

Measured present-day valuesof porosity

Present-day thermal profileversus depth

I

I ]

Measured values ofvitrinite reflectance

4,

__A

->

I I I 1 I I I I

I Initial parameters for basin modelling [ Lithology,sedimentation,erosion,and hiatus, tectonicand thermaleventsin the basement, initial heatflow,boundaryconditions

INumericallsimulationl t

I I

Calculation of rock porosity

L__~I--I

Densityof sedi- ] mentaryand ' basem;t r°cks ' I l Tectonic i L --J subsidenceof | the basement

Restorationof burial historyof the basin

-J;

conductivity, capacity,and generation

,

t

, I t

~t~ Solution of heat transfer equation Temperatureof sedimentaryand basementrocks l

Temperature] profiles versusdepth

[Calculationof] | vitrinite | ~'~. reflectance j--

Time-temperature and burial histories of source rocks Open and dosed pyrolysisdata of sourcesample

V

Kineticparameters estimationfor --I reactions of kerogenmaturation

V --[

Estimateof HCyield history and expulsionthresho,d determinatination

Fig. 6.1. General scheme of our basin modeling program. Arrows with solid lines demonstrate relations between various program units for given variant of basin modeling; arrows with dashed lines show the relation that are involved in correction and calibration of this variant (Fig. 6.1). The first data block contains geological, geophysical, and geochemical data describing basin structure and evolution, including information about present-day sedimentary section, measured values of porosity, temperatures, and vitrinite reflectance. The second block deals with preparing initial parameters for numerical simulation of thermal history of the basin: calculating the volumes of uncompacted sediments on the basin surface, estimating time and amplitude of tectonic and thermal

6.1 - T h e M o d e l s

zo9

events in the basement (thermal activation, stretching of the basement, etc.), calculating the initial temperature profile, and determining temperatures at the base of the computed domain. The third data block uses prepared parameters to carry out a onedimensional numerical simulation of burial, thermal, and geochemical evolution of the basin. The comparison of rock porosities, temperatures, and vitrinite reflectance computed in this block with corresponding present-day values from the first block, as well as the calculated curves of tectonic subsidence, are used to correct the initial parameters for our basin modeling (dashed arrows in Fig. 6.x show the corresponding relationships between these blocks). The third block includes the chemical-kinetic modeling package. Data of open- and closed-pyrolysis experiments are used here for restoring the kinetic spectrum of maturation reactions in source rocks. This spectrum is applied to achieve a numerical estimate of hydrocarbon yield and the expulsion threshold. 6.1.2 Burial and Thermal History Modeling 6.1.2.1

Input Parameters The input parameters for the model include the present-day sedimentary cross section, estimates of the amplitude and rate of erosion, the lithological composition and petrophysical characteristics of rocks, the structure of the lithosphere (basement) and its rock parameters, paleotemperature markers (vitrinite reflectance), paleoclimate, sea paleodepths, present-day surface heat flow, depth-temperature profiles, and information on the paleotectonics and the present-day tectonic setting of the basin. The evolution of the Oued el-Mya Basin is used to demonstrate the model. Table 6.1 presents the basin's main stages of evolution, which include sedimentation, hiatus, and erosion. The input data on basin evolution (see Table 6.1) assume that about z.z km of Silurian-Devonian sediments were eroded during the Permian Hercynian orogeny, nearly the upper limit of erosion amplitude. The presence of thick layers of these sediments in neighboring sedimentary sections supports this assumption. Some details of the problem of erosion amplitude assessment are discussed in the following paragraphs. 6.1.2.2

Burial History When sediments are progressively buried, they are compacted, and pore fluid is expelled. We consider compaction in this program with the following assumptions (Perrier and Quiblier 1974): (1) the volume of solid matrix is preserved throughout compaction and (2) porosity depends only on burial depth and can be expressed as

[1- P((z2)ldg 1 = [1- P(z2) ]dg 2 ,

(6.1)

where P(z) is the porosity at the depth Z, and d Z 1 and dZ~ are the thicknesses of the layer during burial at depths Z~ and Z . respectively. The backstripping procedure for

21o

Chapter 6 •

Burial History and Kinetic Modeling for Hydrocarbon Generation

every discrete sediment layer dZ is based on Eq. 6.1 and the exponential porosity-depth relationship (Sclater and Christie 198o; Deming and Chapman 1989; among others):

P(z)

= Po- z l B

,

(6.2)

where Po is the mean value of porosity on the upper loo-15o m of the sedimentary section, and B is the depth-scale factor. Equations 6.1 and 6.2 are used to reconstruct sedi-

T a b l e 6 . 1 . Main stages of Oued el-Mya Basin evolution ~

No. Stageof evolution

Geologic time Depth (Ma) (m)

1

Sed.

0

-

65

Hiat.

65

-

91

3

Sed.

91

-

93

125-

322

Im, dl, ml

12-18

0-

30

4

Sed.

93

-

97.5

322-

870

hi, an

12-13

30-

80

5

Sed. Sed.

97.5 - 113 113

- 119

125

Surface temp. Sea level (°C) (m)

2

6

0-

Rocktype

125

sn, lm

15

-

t5-

0 18

0

870 - 1042

cl, an

13 - 15

1 042 - 1 489

ct, sn

15

170

80 - 170

7

Sed.

119

-144

1489-2033

cl, sl, dl, ml

15-18

1 7 0 - 130

8

Sed.

144

-213

2033-2886

cl, dl, hl, an, mt

18

130-

9

Sed.

213

- 231

2886-3485

cl, hl, an

18

0 0

l0

Sed.

231

-243

3485-3540

cl, sn, hl

18

11

Sed.

243

-248

3540-3711

vl

18

0

t2

Eros.

248

- 286

2200-2200

-

15 - 18

0 0

0

13

Sed.

286

-360

3711 - 3 7 1 1

cl, sn

8 - 15

14

Sed.

360

-408

3711 - 3 7 1 1

cl, sn

7-

8

0-240

15

Sed.

408

- 428

3711 - 3 8 5 4

cl, sn

5 -

7

240 - 350

16

Sed.

428

-438

3854-3924

cl, sn

5

350

17

Sed.

438

- 590

3 924 - 4100

cl, sn

5 - 15

350 -

0

a Depth column shows present-day depths of the bottom (first number) and roof (second number) of the sedimentary layers; that is, the erosion amplitude. Sed. = sedimentation, Hiat. = hiatus, Eros.= erosion, an = anhydrite, cl = clay and shale, dl = dolomite, hi = halite, lm= limestone, ml = marl, sl = siltstone, sn = sandstone, vl = volcanics, No. = number o f the basin's evolution stage.

Fig. 6.2. Burial and thermal histories of the sedimentary section, Takhoukht region, Oued el-Mya Basin. a Pateodimate history based on literary paleogeographic data of the regeon, b Burial, thermal, and maturation histories resulting from basin modeling. Note that the considerable rise of isotherms in the post-erosion period is related to the Permian-Triassic thermal activation in the lithosphere. More moderate activation occurred here in the Cretaceous-Cenozoic. The temperatures of Silurian rocks did not exceed 85 °C during the pre-erosion period despite significant amplitude of erosion, c Tectonic subsidence of the basement surface calculated in local isostasy approach by removing of sediment and water load (solid line) and by consideration of variations in densities of basement rocks (dashed line). The coincidence of solid and dashed curves provides an additional opportunity to control the sequence of the tectonic and thermal eventes in the lithosphere. STRi and STR2 = streching periods of the basement; TACt and TAC2 = periods of thermal activation in the basin lithosphere, d Variations in sedimentation (>o) and erosion (I

i

~

i

i

i

m ~

~

238

Chapter 6 , Burial History and Kinetic Modeling for Hydrocarbon Generation

,:--~

~

,-,... o-~

"~.,

"~ I

I

b~

E

E

E °

o "~

~, ~

"~ 8

I

[

I

'~"

,o

I

I

I

>o

%

I

I

.-~

m

m

.~,

~

,~

o

~

~,_

-~ ,~

m

8 o

=

=>-

°

~

%'.E

m

I •

o=

I

E

E

._

~ ~o ~

-~

~

~

~"

IU o

u,

U

t m

"U o ~.D

°

._~

~

_s

°_=8:=

(u

.

~

~:

.

~~

~

~

=o ~. . . . . .~. . _8

tu

~

~_~

,_

.

.

.

6.2 - A p p l y i n g t h e M o d e l t o Saharan Basins

239

the Cambrian along North Africa when there was a progressive marine transgression, which continued into the Ordovician (Burollet 1989; Klitzsch 199o). Uplifts, such as the Hoggar and Reguibat, did not exist at that time; they rose after the Ordovician. Meanwhile, the Tindouf, Reggane, Ahnet, Mouydir, Ghadames, Tilizi, Murzuk, and Kufra (in Libya to the east) intracratonic basins subsided (Petters 1991). Gondwana drifted over the South Pole during the Ordovician (Neugebauer 1989), and by the Late Ordovician the South Pole was located far inland in northwestern Africa, leading to widespread continental glaciation. By the Middle Ordovician, interlayered shallow-marine sandstones and argillites had been deposited. Dark argillites, consisting of micaceous graptolitic and trilobitic clays, were deposited during an extensive marine transgression presumably due to melting of the Saharan glacial cap (Beuf et al. 1971; Rognon 1971). No substantial interruption in deposition between the Ordovician and Silurian is observed. Repeated marine transgressions during the Early Silurian resulted in the deposition of thick, dark graptolitic clays, argillites, and sandstones. The Ghadames and Oued el-Mya Basins, which have high potential for hydrocarbon generation, were major argillite depocenters at this time. Early Devonian continental sediments with plant remains discordantly overlie the Silurian strata. Alternating sequences of clays and sandstones reflect an extended cycle of alternating marine transgressive-regressive deposition during the Devonian. Devonian shales, particularly the Middle and Upper Devonian, and Silurian shales are considered to be the principal source rocks in the Saharan basins. Carboniferous shales are also considered to be good source rocks. General uplift resulting from the Hercynian orogeny led to a major withdrawal of the sea (Bishop 1975). The dominant feature of post-Hercynian erosion on the Saharan Platform is a T-shaped anticlinorium that extends from Algeria into Tunisia. To the east, a projection of the Hercynian Nefusa uplift of Libya extends westward and connects with this anticlinorium. The absence of Permian sediments in the Algerian region suggests that this area remained uplifted. Marine transgression taking place at this time resulted in the deposition of thick Permian marine sediments in Tunisia and Libya to the east. These Permian shales form seals to Silurian sandstone reservoirs in the Libyan oil fields. Restriction of the western Tethys basin and post-Hercynian subsidence along the margin of the African landmass led to a new cycle of sediment deposition, which included a thick series of Triassic and Liassic evaporates. This two-stage history influenced source rock and reservoir rock diagenesis. Triassic sediments are widespread in the northeastern part of the Saharan Platform, namely in the Ghadames, Oued elMya, and Trias basins, as well as in a part of the northern flank of the Illizi Basin. Triassic fluvial and shallow-marine sandstones commonly overlay the surface of the Hercynian unconformity. Triassic andesitic and basaltic flow's are abundant in the Triassic section and commonly overlie Cambrian and Ordovician sandstones above the Hercynian unconformity, forming a good seat. Volcanic activity at this time suggests a thinned crust and thermal activation related to the Hercynian orogeny. The Middle Jurassic and younger section on the stable Saharan Platform is dominated by relatively thin lagoonal dolomites, evaporates, and shales. Cretaceous sediments consist of alternating evaporates, limestones, dolomites, and thin layers of sandstone. Aptian-Albian nearshore carbonate facies are oil-bearing in Tunisia. Tertiary sedimentation is particularly present in Tunisia and farther over a wide scale of thickness (up to 7 ooo m). These sediments occur from the Paleocene to the Pliocene in the east, and

240

Chapter 6 • Burial History and Kinetic Modeling for Hydrocarbon Generation

in the nearshore areas of Gabes Gulf on the Tunisian Mediterranean coast. The Tertiary Alpine orogeny uplifted the unstable part of the platform and formed a multitude of folds and complex structures. The Atlasides folded belt forms the northern province. 6.2.2 Oued eI-Mya Basin 6.2.2.1

Tectonic Subsidence and Thermal History The burial and thermal histories of the northern Oued el-Mya Basin are presented in the model. Two methods are used to calculate the relative change in tectonic subsidence that determines the sequence of thermal and stretching events in the lithosphere (Fig. 6.2). A brief description of this sequence follows. Slight variations in the amplitude of tectonic subsidence from 6oo to 48o Ma indicate only moderate variations in heat flow during this time. This reflects slow cooling of the basement lithosphere from a thermal state having an initial heat flow of about 52 m W m -a. Basement subsidence from about 4oo to 35o Ma accompanied the deposition of about 2 50o m of clays and sands, and involved basement stretching with an amplitude of about 1.2 for 95 Ma (Fig. 6.2). Slow stretching rates resulted in a Moho depth change rather than a change in isotherm depths. Sinking of the isotherms at about 49o Ma was due to climate cooling, which continued up to the Early Carboniferous. The subsequent rise of the isotherms at 49o-35o Ma was due to the transition from low-temperature gradients in the basement (high thermal conductivity) to higher temperature gradients in the sedimentary cover (low thermal conductivity). Devonian sedimentation was followed by an interruption that lasted throughout the Carboniferous. The subsequent Hercynian orogeny resulted in uplift and erosion of the northeastern part of the basin, including the Takhoukht region. We estimate that about a 2oo m of Devonian and Silurian sediments were eroded. Thermal activation of the lithosphere in the northern Oued el-Mya Basin began in the Late Carboniferous (28o Ma) (Fig. 6.2). Thermal diapir uplift occurred at an average rate of about 5-5 km Ma-1 for a period of lo Ma Diapirs remained immobile for 35 Ma at a depth of less than 3o km. Surface heat flow reached 9o mW m -2, which is close to the values observed in present-day continental rifts (Smirnov 198o). The presence of relatively thick Triassic volcanics in the Oued el-Mya Basin is evidence of high thermal gradients in the Permian-Triassic. Subsidence of the basement in the Middle Triassic was a consequence of rapid cooling of the anomalously warm basement. Rapid deposition of salts and anhydrides, with their high thermal conductivity, also contributed to the sinking of isotherms in the Jurassic and the Cretaceous. In the Early Cretaceous, deposition was accompanied by stretching of the lithosphere (stretching amplitude about 1.2), which lasted to the end of the Cenomanian (Fig. 6.2). This second stretching phase accounts for the subsidence of the top of the basement during the last thermal activation of the lithosphere, which began in the Berriassic (145 Ma). The last thermal activation was accompanied by uplifting of the thermal diapir's roof at an average rate of 1 km Ma -1 for approximately 2o Ma in the Aptian and Albian. This roof remained at a fixed depth of about

6.2 • A p p l y i n g t h e M o d e l t o Saharan Basins

zO

60 km from the Albian to the present. The last thermal event explains the rather high temperature gradients in the present-day sedimentary section, which contains thick evaporates and a relatively significant level of maturation in the Lower Silurian rocks (Ro= o.7o-o.8o%). The rising of isotherms in the Lower Cretaceous is related to this heating event, as well as to the deposition of low-conductivity sediments; however, the deposition of 8oo m of sak during the Albian and Cenomanian resulted in short-term sinking of the isotherms. Slow sedimentation during the Cenozoic resulted in only minimal variation in the depths of isotherm and heat flow. The relatively high value of this heat flow (about 6o mW m -2) is in accordance with the high value of the presentday thermal gradient in the salt-bearing sediments of the northern Oued el-Mya Basin. The calculated present-day temperatures correlate well with the temperatures measured at 3 739, 3 785, and 3 989 m in boreholes. 6.2.2.2

Source Rocks

The principal hydrocarbon source rocks of the Oued el-Mya Basin are Silurian (Gothlandian) and Devonian (Ernsian, Givetian, Frasnian, Famennian) shales and, to a lesser degree, Ordovician shales (E1-Gassi and Azzel formations). The present areal distribution of Paleozoic (Ordovician, Silurian, and Lower Devonian) shales is a function of their initial distribution and the extent of Hercynian erosion. Maximum initial thicknesses were south, southwest, and west of the basin. Present-day thicknesses range from 6oo to 7oo m in the south to 28o to 66o m in the west and 22o to 46o m in the center of the basin. The Takhoukht section has about 4oo m of shales. The Ordovician shales contain mainly sapropelic, or mixed, organic matter and have an average TOC (total organic carbon) of o.9%. In our thermal model for the Takhoukht region, the base of the Ordovician section reached the main stage for oil generation (Ro = o.7%; TTI (time-temperature index) = 9o) at the end of the Cretaceous (Table 6.5). In the southern part of the basin, the main stage for oil generation could have

Table 6.5. Computed characteristics of the main source formations, Takhoukht area, Oued el-Mya Basin a

Layer

Depth (m)

T (°C)

Ro (%)

TTI

Early Carboniferous ( - 3 6 0 Ma) Ordovician shales Lower Silurian shales

2 438 - 2 635 2 359 -- 2438

84 82,5 -

90 84

0.490 - 0,526 0.480 - 0.490

4 3-

6 4

70 65 -

90 70

End o f the Mesozoic (~65 Ma) Ordovician shales Lower Silurian shales

3 743 - 3 922 3672 - 3 743

107.3 - 102.7 101 102.7

0.670 - 0.700 0.660 - 0,670

3924-4t00 3854-3924

103.2 - 108 101.2 - 103.2

0,735 - 0 . 7 6 7 0.723 - 0 . 7 3 5

Present Ordovician shales Lower Silurian shales

131 - 159 1 2 8 - 131

a T - t e m p e r a t u r e ; R0 - vitrinite reflectance calculated by kinetic model of vitrinite of Sweeney and Burnham (1990); TTI = time-temperature index (Lopatin 1971 ; Waples 1980).

242

Chapter 6 . Burial History and Kinetic Modeling for Hydrocarbon Generation

Fig. 6.14. Hydrocarbon (ttC) yields (solid line), rates of hydrocarbon generation (dashed line), and expulsion threshold in the geological history of the Silurian source shales of the Oued el-Mya Basin. Calculations used time-temperature history of the Silurian rocks and activation energy spectrum shown in Makhous et al. (1997). Two stages of hydrocarbon generation took place in the basin history: in the pre-erosion Carboniferous and during the Cretaceous-Cenezoic

400 300 O"

300

Time (Ma) 200

HC yield - - - Rate of HC generat on

0

100

Secons stage /L 6 ofHCgeneration / t v . ~

o

200-~

:t

,

,

3}

i l00

0

First stage of HC generation I "

0

been reached as early as the end of the Paleozoic. Today, in the northern part of the basin (Takhoukht area), organic matter at the base of the Ordovician is mature with respect to oil generation (R o = o.73-o.77%; TTI = 13o-16o; Table 6.5). In the southern part of the basin, Ordovician source rocks are in the gas window. Silurian and Early Devonian shales contain sapropelic, mixed and humic organic matter having a TOC range of 1.o to lO.O%. Lower Silurian radioactive shales in the central and northeastern parts of the basin contain up to 16% TOC. Our modeling shows that the onset of oil generation in the Lower Silurian and Early Devonian shales occurred at the beginning of the Cretaceous (Re = o.65%; TTI = 7); peak of oil generation (Re = 0.70%; TTI= 75) occurred as early as in the Albian (Figs. 6.14, 6.15). Today, the Silurian shales in the Takhoukht region are mature (Re = o.71-o.73%; TTI = lOO-13o), whereas in the south they are overmature (Re = 1.25-1.7o%), and peak oil generation occurred in the Paleozoic. The Middle Devonian, Late Devonian, and Carboniferous section in the southwest and northwest of the Oued el-Mya Basin is characterized by high-TOC shales (0.5-2.5%) and by mature to overmature kerogen (R o = o.7-1.5%). Carboniferous shales are considered potentially good sources for gas generation only because of their moderate organic matter contents and restricted occurrences within the basin. The Triassic, Jurassic, and Cretaceous sediments have low organic matter contents (generally TOC = O.l-O.3%) and low maturation level (Re = o.4o-o.5o%); consequently, they have poor hydrocarbon potential.

6.2.2.3 Maturation History and Hydrocarbon Generation in the Takhoukht Area Ordovician Sources (EI-Gassi Formation). Modeling results for the Ordovician shales are presented in Table 6.5. The Ordovician shales have an average hydrogen index (HI) value of ~95 mg HC / g TOC and an average TOC of o.78%. HIIOI (oxygen index) correlations show that the Ordovician source rock's kerogen is a mixture of type I kerogen with the initial potential of 7:o mg HC / g TOC (Espitali~ et al. 1988) and type II

6.2 • Applying the Model to Saharan Basins 0 0

•

1-

~

~ ~,

S

D

¥1

¥

N.~

z43

C i

f

P i

IT

T

J

¥1

¥

K i

¥1

CZ ~

¥

i

~

0.5

0.95 ~ 1.3

4-

1.8 ~ 5-

s90

s40

4;o

44o

3;o

340

2;0

Geological

240

1;0

140

;0

40

0

time (Ma)

Fig. 6.15. Generalized pattern of the main source rocks, burial, generation, and expulsion histories. Solid lines = the main variant with erosion; dashed lines = variant free from erosion. Long dashed lines

represent a stabilization of source subsidence in some areas beginning in the Late Cretaceous and during the Cenezoic.It stands to reason that the achieved maturation level during source subsidence is irreversible, and Ro values do not change in the course of uplifting

kerogen with the initial potential of 630 mg HC / g TOC. Using the paleotemperatures computed for the Ordovician section by our model, we obtain total hydrocarbon yields of 84 mg HC / g TOC for type I kerogen and 493 mg HC / g TOC for type II kerogen. The residual potentials are 626 (type II) and 137 (type I) mg HC / g TOC. We infer that the observed residual potential of z95 mg HC 1 g TOC represents a mixture of kerogen types (about 68% type II and 32% type I). Our calculations show that the hydrocarbon yield from the Ordovician sources was 4o-45 mg HC / g TOC by the end of the Carboniferous, and is 36o mg HC / g TOC today. Hydrocarbon yield during the first Paleozoic stage of maturation represents 12% of the final yield. Silurian Sources. Restoration of the activation energy spectrum for hydrocarbon generation in our program used an example from the Gothlandian source shales in the Takhoukht area (Fig. 6.2). The initial hydrocarbon potential was determined to be HIo = 630 mg HC / g TOC, which is typical for open-marine kerogen (type II) (Espitali~ et al. 1988; Ungerer et al. 199o). The present residual potential of the sources is about 45% of the initial potential, implying that about 55% was generated between about 350 Ma and the present (Fig. 6.14). According to our calculations, hydrocarbon yields were 96% liquids and 4% gas. Less than 0.2% of liquid hydrocarbons were subjected to secondary cracking. The calculated hydrocarbon history is shown on Fig. 6.14, The yield during the Carboniferous (from 360 to 286 Ma), prior to the Permian erosion, accounts for about 6% of the total generated hydrocarbons. A small local peak on the hydrocarbon yield rate curve between 360 and 286 Ma (Fig. 6.14) corresponds to this first stage of hydrocarbon generation. The comparatively low yield rates are associated with moderate tem-

z44

Chapter 6 - Burial History and Kinetic Modeling for Hydrocarbon Generation

peratures (82-85 °C). A second and final stage of hydrocarbon generation occurred during the Campanian (12o-9o Ma), when source temperatures exceeded loo °C. In this stage, the rate of hydrocarbon generation was one order of magnitude higher than the rate of generation during the Carboniferous (Fig. 6.14). The activation energy spectrum for hydrocarbon generation (Fig. 6.2) indicates that two reactions, the first having an/~i (activation energy) of 50 kcal mo1-1 and the second 52 kcal mo1-1, were the major contributors to hydrocarbon generation and account for about 86 and 13% of the yield, respectively. Expulsion from the source is assumed to commence when zo% of the free pore space is saturated with liquid hydrocarbons. For the most organic-rich Gothlandian source shales (TOC = 11.8%), ~o% pore saturation was achieved at the beginning of the Coniacian (about 88 Ma; Fig. 6.14), when the hydrocarbon generation was as high as about 35 mg HC 1 g TOC and source rock temperatures reached 9o-loo °C. The expulsion threshold for shales having TOC = ~4.4% was attained 3 Ma earlier. Mesozoic subsidence was accompanied by formation of new structures at the same time that pre-existing traps were completely or partially destroyed. Hydrocarbons migrated from the destroyed traps and accumulated in traps having Triassic and lurassic seals. Consequently, reservoirs in the basin are either Triassic (Berkaoui, Benkahia, Guellala) or Paleozoic (Hassi Messaoud, E1-Agreb) (Fig. 6.11). Further subsidence of the Paleozoic sources during the lurassic and the Cretaceous caused hydrocarbon generation, which was followed by hydrocarbon migration and accumulation in traps in the northern part of the basin. Both Triassic and Paleozoic reservoirs were filled with hydrocarbons generated in Paleozoic sources. Most of the hydrocarbons generated in Paleozoic source rocks during the ]urassic-Cretaceous and following lateral or, to a greater extent, vertical migration were trapped in Triassic reservoirs. Migration pathways along the Amguid-el-Biod/Hassi Messaoud axis were in a southward direction far from subsided zones to more uplifted zones. The migration of hydrocarbons generated in Devonian shales to more uplifted zones also took place in the east from the Ghadames Basin. 6.2.3 Ghadames and Illizi Basins 6.2.3.1

TectonicSubsidence and Thermal History The first major tectonic event affecting the Ghadames Basin was the Hercynian orogeny. Prior to its onset at the close of the Carboniferous, 2 8oo-3 9oo m of Paleozoic sediments had accumulated. The Hercynian orogeny resulted in uplift and the subsequent erosion of about 9oo m of Late Paleozoic sediments (Fig. 6.15). In the lurassicTriassic, the Ghadames Basin was the center of inversion tectonic movements, which caused subsidence of its northern, northwestern, and western parts; that is, in regions that experienced the most uplift at the close of the Pateozoic (Makhous et al. I995). Evaporates deposited at this time covered an area that stretched far beyond the boundaries of the depression and had thicknesses exceeding 9OO-lOOOm in the Messdar, E1-Khtir, Fort Lalleland, and Keskessa areas. By contrast, there was minimal subsidence in the Triassic-lurassic and an absence of evaporates in areas subjected to maxi-

6.2 - Applying the Model to Saharan Basins

245

mal subsidence in the Paleozoic (Rhourde Nouss, Ektaia, and the southeast area of the basin). Maximal total thickness of all Triassic-Jurassic formations, including detrital sediments and evaporates, is 19oo-190o m in the Wadi-Teh, Haid, and Bir Rebaa areas. In the Cretaceous, subsidence rates in the southeastern, western, and northwestern regions of the basin decreased, and the final configuration of the Ghadames Basin was formed. The Ghadames Basin is a Mesozoic tectonic structure whose closure took place in the Cretaceous. The sequence of tectonic events in the Akfadou area is derived from an analysis of the tectonic subsidence of the basement surface. The sequence basically repeats the sequence determined for the Takhoukht area: stretching through the OrdovicianDevonian, thermal activation during the Permian, and basement stretching in the Early Cretaceous, with thermal activation of the lithosphere from the beginning of the Cretaceous until the present. A high maturation level of organic matter is observed in the lower horizons of the sedimentary section: Ro = 1.oo-ln9% at depths of 3.0 3.9 km. Higher temperatures are characteristic of the sedimentary section of the Akfadou area as compared to the Takhoukht area. Temperatures of about 12o °C were reached in Early Devonian sediments. Our model calculations for the end of the Early Carboniferous (about 33o Ma), and before the Hercynian uplift and erosion, give temperatures and maturation levels for the Ordovician, Silurian, and Early Devonian shales consistent with early to main stages of hydrocarbon generation (Table 6.6). Toward the close of the Carboniferous (about 289 Ma), the sapropelic and humic organic matter of the Ordovician, Silurian, and probably Devonian source shales was sufficiently mature to generate both liquid and gaseous hydrocarbons. Silurian and Devonian shales in other areas of the basin are also characterized by considerable potential

Table 6.6. C o m p u t e d characteristics of the m a i n source formations, A k f a d o u area, G h a d a m e s Basin a

Layer

Depth (m)

T (°C)

Ro (%)

TTI

3220-3440 2 7 8 0 - 3220 2116 --2780 1280-2116

113 - 118 1 0 2 - 113 86 - 102 6286

0.723 - 0 . 7 5 6 0.640 - 0 . 7 2 3 0.510 - 0.640 0,367-0.510

4300 4500 3870-4300 3 270 .- 3 870 2 5 5 0 - - 3 270

t 39 144 128 - 139 114 - 128 96 114

1,111 - 1.t70 0,920 1.111 0,770 - 0.920 0,650 - 0.770

4650-4850 4240-4650 3650-4240 2 9 5 0 - 3 650

146136 121 104 -

1.267 1.082 0.861 0.709

Close of Early Carboniferous ( - 3 3 0 Ma) Ordovician shNes Silurian shales Lower Devonian shales Middle and Upper Devonian shales

42 17 5 0.8-

54 42 17 5

End of the Mesozoic ( - 6 5 Ma) Ordovician shales Silurian Shales Lower Devonian Shales Middle and Upper Devonian shales

920 425 165 42

-1300 - 920 - 425 - 165

] 923 866 323 88

-2775 - ] 923 - 866 - 323

Present Ordovician shales Silurian shales Lower Devonian shales Middle and Upper Devonian shales

152 146 136 121

-

1.378 1267 1.082 0.861

a T = temperature; R0 = vitrinite reflectance calculated by kinetic model o f vitrinite o f Sweeney and Burnham (1990); 3-1-1= t i m e - t e m p e r a t u r e index (Lopatin 1971 ; Waples 1980).

z46

Chapter 6 • Burial History and Kinetic Modeling for Hydrocarbon Generation

to generate liquid hydrocarbons. The relatively moderate Hercynian uplift in the region and consequent erosion slowed down organic matter maturation slightly. According to our modeling, the resulting temperature decrease ranges from ao °C for sediments at the base of the Ordovician to 16 °C for sediments at the base of the Carboniferous. Toward the end of the Mesozoic subsidence (about 69 Ma), organic matter in the Ordovician, Silurian, and Devonian shales was subjected to further maturation, and temperatures ranging from 114 to 144 °C and Ro values of o.8-1.2% were reached (Table 6.6). Ordovician and Silurian source shales realized most of their hydrocarbon potential in the Paleozoic. In the Mesozoic, hydrocarbon generation occurred throughout the Paleozoic section, including Carboniferous shales, in areas of maximal subsidence. During the entire history of sedimentation in the Illizi Basin, the granodiorite basement of the Mereksen area was a horst with an amplitude of some zoo m, but rapidly damping out in Cambrian-Ordovician sediments. In the Triassic and particularly in the Jurassic, tectonic movement caused horizontal stretching of the crust, resumption of movement on old faults, and the generation of new faults. The amplitude of faults within and bounding the horst decreased considerably with time. These processes were accompanied by subsidence of the basement. The Mereksen region is considered to be an old consedimentation structure with a tendency for disintegration, with subsidence of the basement in the Jurassic and at the beginning of the Cretaceous. Prob-

400

6OO

Time (Ma)

200

~2

E OJ

4 Sec bot

5

.......

Bas

.........

Jsol refl

. . . .

lsol

Fig. 6.16. Burial, thermal, and maturation histories of the sedimentary section of the Akfadou region, Ghadames Basin. The greater volume of sedimentation compared with the variant of Oued elMya Basin in Fig. z in Makhous et al. (1997) led to the greater level of maturation of organic matter in Silurian rocks, despite a moderate level of Hercynian uplift

6.2 - Applying the Model to Saharan Basins

247 Time (Ma)

310

510

II0

'\,

Tr-J

"X

oo"......................... A

2

o ~

Ds

S-""

¢-

¢1

..~.....

- o 5 0°/o 4..

"\',

s

.

.

.

.

.

3

° z -

Basement surface

.........

Isolines of vitrinite reflectance

.

"'~..~

"~'Z

,,,"

~ N °!° | a o C : ......]

Sedimemary layers boundaries

-

.......

.

/

.

.

Isotherms

I1//I / / #

1

I

Fig. 6.17. Burial, thermal, and maturation histories of the sedimentary section of the Iltizi Basin in the Mereksen region. Relatively higher maturation level is achieved in the absence of Hercynian uplift in the area

able source rocks in the Mereksen region are thinner than those in the Ghadames Basin due, in part, to slower sedimentation rates (Figs. 6.16, 6.17). Toward the end of the Carboniferous, Paleozoic basal rocks were at a depth of about 3 2oo m. Interrupted sedimentation was the only effect of the Hercynian orogeny. The region has generally higher thermal gradients than the Ghadames Basin (Makhous et al. 1995). The burial history and thermal model for this basin use an analysis of tectonic subsidence vs. time, and are in good agreement with the present temperature gradient and maturity levels estimated from measured vitrinite reflectance. The sequence of tectonic events that correlate with variations in tectonic subsidence of the basement surface includes periods of minor basement stretching and thermal activation commencing in the Early Cretaceous and continuing to the present. This thermal activation in the basement helps to explain the relatively high temperatures observed in the present sedimentary section. Temperatures of lO5-1o8 °C, measured at a depth of 2 776 m in a Mereksen borehole, are in good agreement with the calculated value of lO7 °C at the base of the Middle Devonian layer at a depth of 2 779 m (Fig. 6.17). The relatively high stage of organic matter maturation (Ro = o.7-1.2%) measured in Devonian shales occurring at depths of 2.5-3.1 km in the Mereksen area exceeds the results of our numerical simulation (Ro = o.8% at depth of about 2.9 kin) (Table 6.7). This high stage of maturation could be attributed to water infiltration into outcropped basin strata and to the thermal effect of rising groundwater flows; however, this problem requires an additional detailed hydrologic investigation.

248

Chapter 6 . Burial H i s t o r y and Kinetic M o d e l i n g f o r H y d r o c a r b o n G e n e r a t i o n

Table 6.7. Computedcharacteristics of the main source formations,Mereksen area, Illizi Basina Layer

Depth (m)

R0(%)

T(°C)

TTI

Close o f Carboniferous (~288 Ma) Ordovician shales Silurian shales Lower Devonian shales Middle and Upper Devonian shales

2 493 - 3 177 2213 - 2 4 9 3 2060-2213 1 741 - 2060

97 - 112 90 - 97 8 6 - 90 78 - 86

0,600 .- 0,707 0.535 - 0.600 0,511-0.535 0.455 - 0.511

14 7.7 5.6 2.9

-

47 14 7,7 5,6

End o f the Mesozoic (~65 Ma) Ordovicianshales Silurian shales Lower Devonian shales Middle and Upper Devonian shales

3179-3832 2918-3179 2777-2918 2485 - 2 7 7 7

115 108105 98 -

129 115 108 105

0,814 1,020 0#50 --0.814 0.730 - 0 . 7 5 0 0,670 - 0.730

3179 - 3 832 2918-3179 2 777 - 2 918 2485 - 2777

116 t09106 99 -

130 1t6 109 t06

0,862 0.790 0,760 0.712

266

-

710

160

-

266

125

-

160

75

-

125

Present Ordovician shales Silurian shales Lower Devonian shales Middle and Upper Devonian shales

.- 1,087 -0.862 - 0.790 - 0.760

391

--1209

229

-

391

191

-

229

109

-

191

a T = t e m p e r a t u r e ; R0 = vitrinite reflectance calculated by kinetic model of vitrinite of Sweeney and 8u rnham (1990); TTt = time-temperatu re index (Lopatin 1971; Waples 1980).

Slow, continuous sedimentation during the period beginning in the Permian resulted in minimal variation in isotherm depths and in the depths of Ro isolines (Fig. 6.17) in the Mereksen area. Our calculations of temperature and organic matter maturity in Ordovician, Silurian, and Devonian rocks at the close of the Carboniferous (approximately 288 Ma) are shown in Table 6.7. These calculations show that at the beginning of the Permian, organic matter in Ordovician and Silurian shales was in the lower part of the oil window (Fig. 6.17). Further subsidence contributed to a temperature rise and further maturation of organic matter. At the close of the Mesozoic, temperatures and maturation indicators in the Ordovician, Silurian, and Devonian shales were loo-13o °C, Ro = o.7o-l.oo%, and TTI= 7o-7oo (Table 6.7). These values suggest that at the close of the Mesozoic, no potential remained for liquid hydrocarbon generation in the Ordovician and some of the Silurian shales, whereas the Famennian shales were at peak generation conditions. Present-day temperatures, Ro, and TTI in the Ordovician, Silurian, and Devonian layers (Table 6.7) suggest that today Early Ordovician shales at a depth of about 3 8oo m are generating dry gas, and the top Ordovician shales are generating wet gas and condensate. The generation of liquid hydrocarbons in the Silurian, Early Devonian, and Middle Devonian shales to a large extent must be exhausted. 6.2.3.2

Maturation History and Hydrocarbon Generation in the Ghadames Basin The sediments in the Ghadames Basin in the Akfadou region were subjected to a higher thermal regime with reduced erosion amplitude and absence of evaporates in the geological section compared to the northern Oued el-Mya Basin in the Takhoukht region.

6.2 , Applying the Model to Saharan Basins

249

Ordovician shales (E1-Gassi and Azzei formations) in the Ghadames Basin and southern part of the Trias basin have a TOG range of o.5 to 1%. Ordovician shales in the north of Illizi Basin have TOC contents that average 1.3%. At present, the amorphous Ordovician organic matter is in the main phase of oil generation. Silurian (Gothlandian) shales in the Ghadames and Illizi Basins have TOC concentrations ranging from o.5 to 2.0%. The kerogen is amorphous and presently has no remaining oil potential. Areas with the higher TOC generally correspond to primary depocenters and to a moderate level of maturation. There are numerous areas where the Silurian shales have lower TOC and overmature organic matter. The maturation history of Silurian sources in the central part of the Ghadames Basin, including the Akfadou region, was only minimally affected by Hercynian erosion. Our paleotemperature calculations indicate that these sources generated petroleum as early as the Early Carboniferous (Fig. 6.16). Since the Hercynian uplift in the central part of the basin was minimal, the appropriate conditions for hydrocarbon generation were not interrupted (Fig. 6.16). Geochemical data show that these currently overmature Silurian source shales initially had considerable oil potential. Their estimated initial potential for hydrocarbon generation in the Ghadames Basin can be considerably higher than that estimated only on the basis of mean content of the present-day overmature organic matter, which is low. The Ghadames Basin contains thick beds of Devonian shales containing oil-prone amorphous organic matter and TOC concentrations ranging from o.5 to 5.o%. This organic matter is mature to overmature (R o = o.7-1.6%) in the Upper and Middle Devonian shales and often overmature (R o = o.8-2.0%) for Lower Devonian shales. These shales are considered to be excellent sources of liquid hydrocarbons. Middle and Late Devonian source shales in the central part of the Ghadames Basin are currently within the oil- or gas-condensate windows at temperatures of ~oo-~1o °C ('fable 6.5, Fig. 6.16). Our model calculations of the quantities of hydrocarbons generated and the generation rate of hydrocarbons in the Late and Middle Devonian shales of the Akfadou region are shown on Fig. 6.18. About 9o% of the initial hydrocarbon

Fig. 6.18. Hydrocarbon yields

(solid line), rates of hydrocarbon generation (dashed line), and expulsion threshold in the geologicalhistory of the Middie and Late Devonian source shales of the Ghadames Basin, Akfadou region

400 300

Time (Ma) 200

300

I

HCyield

]

8

/

- - - Rate o f HC ~

o

0

100

I ', i

.~

i

I I

I

6~

400

~o

U =1=

g U "r

~ 2oo 0

E

U

/

4"1" E

\

0

250

Chapter 6 - Burial History and Kinetic Modeling for Hydrocarbon Generation

potential has alreadybeen generated. The rate of hydrocarbon generation has a clearly defined peak in the Late Cretaceous, when formation temperatures reached 11o °C (Figs. 6.16,1.18). The decrease in the generation rates from 8o Ma to the present is because most of the generation is controlled by reactions having tow activation energies (50 and 52 kcal mol-1). Late and Middle Devonian source shales having an average TOC of 4-5% reached the expulsion threshold at the end of the Aptian (115 Ma). Gas products could account for less than 5% of total hydrocarbon output. For comparison, we give modeling results for a standard spectrum of kerogen type II with initial potential HIo = 63o mg HC/g TOC (Espitali~ et al. 1988). About 7o% of this potential was realized during the burial history of the formation. The expulsion threshold was reached in 7o Ma. The expulsion threshold for liquid hydrocarbons was attained in Campanian time (nearly 8o Ma) for Late Devonian source shales in the Mereksen region. During the Paleozoic, only some Ordovician and partially Silurian shales could realize their petroleum potential, whereas during the Mesozoic, the process of hydrocarbon generation occurred throughout the Paleozoic shales, including Carboniferous shales in the areas of their maximal subsidence (Fig.6.17). Carboniferous shales in the Ghadames and Illizi Basins contain gas-prone humic kerogen and have TOC ranging from i to 4%. Because the thickness of Carboniferous shales is considerable in this province (from 50o to 15oo m in its central part), they may be considered as a good source for hydrocarbon generation. Present-day vitrinite reflectance ranges from o.so to o.71% (TTI= 7-11o; Tables 6.6, 6.7). 6.2.3.3

Maturation History and Hydrocarbon Generation in the Illizi Basin Devonian shales in the Illizi Basin have properties similar to those in the Ghadames Basin but are thinner. According to our model, Middle and Late Devonian sources in

Fig. 6.19. Hydrocarbonyields (solid line), rates of hydrocarbon generation (dashed line), and expulsion threshold in the geological history of the Middle and Late Devonian source shales of the Itlizi Basin, Mereksen region

400

300

Time (Ma) 200

1 O0

4"g

~400

o~

I

~: 300

g

illi~lj

U O

0

200

I I I

O

E t00

-

0

6.2 • Applying the Model to Saharan Basins

251

the Mereksen area began to generate liquid hydrocarbons in the Permian (Fig. 6.17). Considering that the average TOC = 4.5% and the average measured S~= 7.5 mg HC / g rock, the residual potential should be H I = 167 mg HC / g TOC. This value differs from the modeled value by 6%. Total hydrocarbons output during the geological history of these source shales is nearly 75% of the initial potential (63o mg HC / g TOE; Fig. 6.19). In the Trias basin, the Devonian shales have low TOC and are not hydrocarbon sources. The initial and present-day TOC contents of the Devonian shales and their distribution in the Illizi and Ghadames Basins differ greatly from those of the Silurian. Late Devonian shales in the Ghadames and Illizi Basins contain the largest quantities of TOC (2-8%),which is considerably higher than in the corresponding Silurian shales (usually 2%); however, the organic carbon content in the Late Devonian shales decreases westward of the Saharan Platform. These variations are probably linked with the change of direction and the quality of detrital material in the Devonian as compared to Silurian shales. The Hoggar massif to the south was the main source of detrital material during the Early Silurian, whereas the Tihemboka-Zarzaitine-Alrar anticlinal system to the east was a dominant detrital source during the Late Silurian and Early Devonian. During the Ardenian orogeny, local highs originated in the Ghadames and Illizi Basins. These highs contributed detrital material to the basins during the Middle and Late Devonian. 6.2.4 Southern and Western Basins

The thermal histories of the Ahnet, Mouydir, Timimoune, Tindouf, Reggane, and other basins were also modeled. The Ahnet, Mouydir, and Timimoune basins had high hydrocarbon generation potential. In the Sbaa sub-basin of the southern Timimoune basin, the initial TOC content of Silurian shales (9%) far exceeds present-day values (3%). This area, characterized by a relatively moderate level of organic matter maturation (Ro = o.9-1.o%), generated substantial oil. Devonian shales, particularly Middle Devonian shales, have hydrocarbon potential similar to the Silurian shales, Silurian source shales (in the Ahnet, Mouydir, and North Timimoune basins) have relatively high TOC (2%) concentrations; however, the maturation level is high (Ro = 1.2-1.6%), and these shales are likely to be generating gas. Measured TOC ranges from 2 to 8% in the Middle Devonian shales, and from 1 to 5% in the Late Devonian shales, but with maximum decreasing concentrations westward from 1.5 to 5.0% in the Illizi Basin, 1.o to 3.5% in the Mouydir basin, and 1.o to 1.8% in the Timimoune basin. These variations are likely linked to changes in transport direction and in the provenances of detritus as compared to Silurian shales. The level of organic matter maturation in the Devonian sources in these basins is higher (Ro = I-4%) than it is in the equivalent sources of the Triassic province (Oued el-Mya, Ghadames, and Trias basins) due to differing sedimentation, burial, and tectonic histories. Consequently, gas generation is currently expected to occur in the south and west of the Sahara, with the exception of the Sbaa sub-basin, which is at a lower maturation level.

25z

Chapter 6 - Burial History and Kinetic Modeling for Hydrocarbon Generation

6.3

Summary and Conclusions Our modeling has shown that, in a number of basins - in particular, the Ghadames, southern Oued el-Mya, and Sbaa - the initial total organic carbon values for the present-day overmature kerogen (which is beyond the hydrocarbon generation maximum) exceed appreciably the present-day average content of residual total organic carbon (5% in the Upper Devonian shales of the Ghadames and Illizi Basins, 2.5% in Silurian shales in the northern Ghadames, and about 2.5-3% in the Silurian shales of the Sbaa sub-basin). Presumably, these areas were abundant as hydrocarbon generators. The Silurian source shales, despite their present occurrence mostly in the gas window, were not heated sufficiently high in the north Oued el-Mya Basin and in certain areas of the Ghadames Basin. The Devonian source shales of the Ghadames Basin also may be mentioned in this connection. This apparent inconsistency is attributable to erosion of a significant part of the Paleozoic sediments as a consequence of the Hercynian uplift. In the Ghadames Basin, Hercynian uplift amplitude and the extent of erosion are appreciably louver than in the Oued el-Mya Basin, and for this reason the apparent disagreement between the present-day temperatures and the organic matter maturation level is less contrasting. In the south and west Illizi Basin, the deepest subsidence occurred prior to the Hercynian uplift. The moderate Hercynian uplift in these areas entailed no substantial temperature drop; consequently, the organic matter maturation proceeded, although at a slower rate. The resultant effect is that the measured maturation level is higher than expected for the present-day temperatures. Consequently, regional average estimation of the initial total organic carbon requires evaluation of the Hercynian uplift amplitude and the extent of erosion of the Paleozoic sediments for each particular area - and their effect on the kerogen maturation. It would be of interest to determine also the original depocenters, because areas with overmature organic matter exhibit a lower content of total organic carbon in consequence to depletion of the major kerogen potential. High geothermal gradients, in the large majority of cases, are associated with zones of uplifted basement, most notably in the Hoggar massif and Ougarta chain adjacent to the Triassic province's southern and western basins. High geothermal gradients increase along the Amguid el-Biod ridge; relatively low geothermal gradients are typical in the northern Oued el-Mya and Ghadames Basins, where thick Mesozoic evaporates were deposited. In the Pateozoic, favorable conditions for hydrocarbon generation and accumulation occurred mainly in the south and in the southwest of the province. As to the central and northern regions, the generation of hydrocarbons would have taken place preferentially in the Mesozoic (Fig. 6.2o). Promising traps or advantageous structures are those located close to the subsidence zones, where the Silurian and Devonian source shales escaped uplifting and, consequently, the thermal pause, as well as the erosion. In particular, the Ghadames and Illizi Basins constituted a favorable province because they were active during the course of both the Paleozoic and Mesozoic. At the end of the Cretaceous, the eastern Sahara started generating gas.

6.3 • Summary and Conclusions

z53

r

Ir Traps formed 11 I1NorthOuedMya li after migration Ii basin (areaBST)

Cenozoic-Mesozoic (J-K) oil (gas) II generation ~.

Traps formed [_before m grat on ~ Berkaoui Ben Kahla EastOued Mya

T Traps buried

Tilrhmet Hassi R'Met

Hercynian orogeny (unconformity)

/ Destroyed U~ fields /J

I~ -Traps-h~y l! /

~

/ Paleozoic (D3-C) 11 lJ oil (gas) generation I~ "~

.

\ \ \

~up fred . t~nnet, ~, • .' especa ~ n-7 yI~~-entre L. in Late rz_ -J North and ~ ~ EastSahara, [] Tramsformed +lnl-o,uye structuraltrend I I . . IJ . . .laDanKort, Amguid-Hassi DeTore mlgratlon Edjelen, ~o~.,A ~ [ ~ ~ Zarzaitin G'"~a~lame~s,

Daharregion

~I " ~ l ~ w I ~ I =!I. Irapslormect ~ ~

I~

[ I T r a p s moderately]] auellala, /flluplifted. especiallyll DraaTamra, [I inLatePZ ]IEIAg reb

Ahone;clirU bYtectonicinversion ~me~Un' - hypsometric denivellation Y - paleostructuresin ~ North-lllizi basin

Fig. 6.20. Generalizedmodelofpossibleoil-gasgeneration,migrationpathwaysandtrapping(specified examples)

Chapter 7

Degree of Preservation of Hydrocarbon Accumulation as Indicated by Carbon Isotope Analysis

In various regions of northeastern Algeria like Djebel Hamra (Ain-Rich Concession), S4tif, Mddjoun6s, Ain-Touta (Batna) and Djidjet indications of petroleum have been observed. Geochemical and tithological studies carried out previously (Makhous 1982) showed clear and convincing evidence for a secondary migration of hydrocarbons located in deposits within Upper Jurassic and Cretaceous (?) rocks. This region is characterized by an intense development of tectonic dislocations which served as pathways for the secondary migration mentioned, during which parts of these accumulations have been destroyed. As an unresolved problem we still have to delineate the extent of this secondary migration and to evaluate the volume of that part of the hydrocarbons that has been transferred through the zones of dislocation and then destroyed on the way. In other words, we have to find out to which degree the accumulations ultimately trapped in the study zone have been preserved. The analysis of the fractionation of the stable carbon isotopes 12C and 13C in carbonate rocks and organic matter extracted from them allows us to delineate the extent of the secondary processes connected to the migration of hydrocarbons and to undertake an attempt at defining the scale of destruction of potential hydrocarbon deposits. An additional objective of this investigation is to present an actualization of the isotope studies and of their applicability to prospecting for hydrocarbons. The importance and reliability of the information gleaned from carbon isotope studies will be outlined, in particular for tracing one of the prime aspects of petroleum research, viz. secondary migration. 7.1

Methods Employed The results of carbon isotope analyses are expressed as a ratio against the so-called PDB, i.e. calcite from a Cretaceous"Belemnitella americana" with a (13C/lZC)PDB= 88.99 (Craig 1957), according to the following formula: 13 12 13 12 ~3C = ( C/ C)sample- ( C/ C)pD B X103 (13C]12C)PDB

The oceans represent a basin connected to the sedimentation of carbonate rocks. If carbonate sedimentation in it takes place in equilibrium, then its carbon will be enriched in 13Cby 4%o against that of the bicarbonate dissolved in the same waters

2.56

Chapter 7. Degreeof Preservation of Hydrocarbon Accumulation

(Keeling 1968). The majority of the carbonate of marine origin exhibits a fi13C ranging from +6 to -3%o (Park 1960). The decomposition of organic matter leads to the liberation of relatively large amounts of"light" CO2 which will be mixed with the bicarbonates dissolved in water, leading to the formation of carbonates with a pronounced negative fi13C.~'Light" carbon in carbonate rocks may result from the oxidation of organic matter, the oxidation of methane, or the supply of the latter from deeper horizons. With such methane is associated the supply of carbon with fi13Cvalues ranging from -10 to -15%o. It has been noted that microbiological reduction of hydrocarbons by sulfate-reducing bacteria will lead to metabolic carbon dioxide enriched in 1~Cby 5-10%o (Thode 1958). As a consequence of the phenomena described above we will obtain "light" carbonates, the volume of which depends directly on the intensity of the reduction of sulfates or the oxidation of organic matter under participation of the migrated hydrocarbons. The presence of thick carbonate beds of chemical derivation enriched in light carbon isotopes serves as an indication of the oxidation by sulfate-reducing bacteria followed over time by the destruction of vast amounts of petroleum. The smaller the volume of such ~light" carbonates, the better the preservation of petroleum. In other words, a weak development of such carbonates will be a good indicator of the preservation of petroleum deposits (Makhous 1973).This criterion was employed by us during prospecting activities for petroleum and native sulfur in the Mesopotamian basin of Syria and Iraq.

7.2 General Data on Carbon Isotope Composition of Sedimentary Rocks (Carbonates) and Organic Matter from Northeastern Algeria 7.2.1 Carbon Isotope Composition of Carbonate Rocks The carbon isotope composition of carbonates at different levels at Djebel Hamra (Ain-Rich Concession), S~tif, M6djoun6s, Ain-Touta (Batna) and Djidjel was studied in detail. The deposits investigated are enclosed in rocks of the Upper Jurassic and Cretaceous at Djebel Hamra (HM-I) and in the Senonian and Paleozoic in the S4tif and M6djoun~s areas (Fig. 7.1). The mean isotopic compositions were calculated for micritic carbonate rocks not affected by secondary transformations for the various regions mentioned. The Senonian and Paleozoic micritic carbonates of the S6tif and M4djoun~s areas exhibit mean 6t3C values of -o.2 and -0.1%o respectively. The mean for the micritic carbonates of the Upper Jurassic at Djebel Hamra is +1.5%o. For the Ain-Touta and Djidjel areas the mean values of S'3C are -0.4 and -o.6%o respectively.

7.2.1.1 The Djebel Homra Region (HM- I) and AYn-Rich Concession We have already referred to the vertical migration of hydrocarbons within the rocks of the lower part of the succession in the Djebel Hamra region (HM-1) between 2 ooo and

7.2. General Data on Carbon Isotope Composition of Rocks and Organic Matter

E

~ ~~

~~

257

o

S

a o ©

o

8 E~

m

~

o

m~

0~.~

0.~

~ 8~o "~

o

o

~8 8~ ~ °8~ Z ~

Z

U

258

Chapter 7 • Degree o f Preservation of Hydrocarbon Accumulation

Table 7.1. Isotope composi-

t/on of carbon in carbonate rocks and in organic matter at Djebal Hamra region (HMq)

Depth (m)

6~C carbonates (%0)

6~3C organic matter (%0)

801.0 802,0 803,2 893.0

0.0 +2.8 +3.2 -46

-22.2

1334,4- 1 335.0 1429.0- 1430.3 1913.4- 1913.8

+4.5 -7.4 +0.6 -5.8

-21.9 -26.2 -22.3 -24.8

2411.0 -2414,0 2 683.2 - 2 686,4 2 753.0 --2 754.2 3 474.0 - 3 475.0

+1.5 + 1.7 +1.5 -6.1

-22,5 -22.2 -25.8

3526.0-3531.0

+2,3

36175 -3621.7 3 940.0 - 3 942.5 4 067.0 - 4 069.0

+1.5 +1.4 +0,9

800.0801,0 802.0892.0-

1493.0 - 1 494.3

-25.6

-24,0

4 069 m. The carbonate carbon of these rocks exhibits a truely homogeneous isotope composition of +o,o to 4.5%0 with the exception of three samples with 6'3C of -4.6, -6.1 and -7,4%o (Table 7.1, Fig. 7.2). Such an isotopic composition is characteristic of marine carbonates. We may thus suggest that during the unidirectional process of hydrocarbon migration the really quite limited recrystallization of the carbonates has not caused a substantial change of the carbonate carbon isotope composition of these rocks, although there are convincing indications for pathways of hydrocarbons in the sediments. In the upper part of the succession in drill hole HMq the carbonate carbon isotope composition fluctuates between well-defined limits of +4.5 and -7.4%0 (Table 7.1, Fig. 7.2). In this case, recrystallization of carbonates and neoformation of calcite or dolomite have taken place under participation of CO 2 which in certain instances contained a large portion of isotopically light carbon (12C)whereas in other instances there was a predominance of hea W carbon (13C). The results obtained, together with other geochemical data, show that in the zones in which the sedimentary rocks have been affected by infiltration waters the migrating hydrocarbons have been subjected to oxidation and other changes, to be discussed in detail below. 7.2.1.2

The $~tif and M~djoun~s Areas For samples taken up-dip of the Sdtif and M4djoun6s areas the 613C values range from +2.9 to -5.8%o (Tables 7.2, 7,3; Fig. 7.2). The "light" carbon makes itself felt in the carbonates where we find traces of recrystallization or of other mineralogical neoformations and in particular the appearance of mixed-layer clay minerals with perfectly ordered crystal structures. In the same samples microfissures are filled by secondary calcite or dolomite with detrital carbonate cement frequently being present between the crystals. In other cases we observe entire fields of neoformed minerals (dolomite

7.2 • G e n e r a l D a t a o n C a r b o n

-~9

-8

-7

~6

-4

-5

Isotope Composition of Rocks -3

-2

-1

+t

~ ~

and Organic Matter

+3

+2

~

+4

~ ~

+6

+5

~

z59

+7

.........

+8 8~C (%o)

DJEBELHAMRA

MEDJOUNES

AINTOUTA (SATNA)

.........DJIDJEL

.........MARINE

CARBONATES

. . . . . . . . . . FRESH~WATER CARBONATES

-9

--8

Fig. 7.2.

-7

q6 _ -S,

-4

~3

52

-I

+I

+2,

+3,

+4,

+5,

+6

+7, ~8

8~C

(%o)

Isotopic composition of carbonate carbon

Table 7.2. Isotope composition of carbon in carbonate rocks and in organic matter

(OM) in the Setif

region

No. of 613Ccarbosample nates (%0)

613Corganic Core(%) matter (%o)

No. of 61~Ccarbosample hates (%0)

6~3Corganic Corn(%) matter (%0)

6a 6b 7 8 9 10a 10c 10d l 1a 11b 11 c 11 d 12 20 21 22 23 24b

-25.01 - 22.2 -25.1 -

25 26c~ 26b 34 35 36 39a 39b 56 57 58 59a 59b 60 61 62 62a 62b

-24.4 -24.1 -

-5.6 2.0 2.0 0,1 2.9 -1.0 1.9 -0.6 -1.3 -1.5 - 1.8 -3.4 -0.9 1.8 2.0 0.1 0.1 -0,3

.-

- 23.8 -24.2

0.82 0.61 0.14 0.9 0.24 0,43 0.31 0.72 0.42 0.66 0.31 0.12

-0,9 -I A -1,2 2,1 1,5 0~1 -2.6 0.4 -0.6 -3.9 -5,2 -4.7 1,3 -1,7 0,5 -0,2 -0,9 -0A

-23.3 -23.8

0.86 1,46 0.75 0,09 0.10 0,09 0.10 0.11 0.12 0.14 0,64 0.80 0.27 0.13 0.18 0.25 025 0,31

260

Chapter 7 .

Degree of Preservation of Hydrocarbon Accumulation

T a b l e 7.3. Isotope composition of carbon in carbonate rocks and in organic matter

(OM) in the re-

gion of M~djounds

No. of 613Ccarbosample nates (%0)

613Corganic Corg(%) matter (%0)

No. of 613Ccarbosample nates {%o)

613c organic matter (%0)

Corg(%)

Io

0.I

-

1.08

30b

-0.3

-

Ib

-0.2

-

0.96

31

- 1.6

-

0.48

2a 2b 2c 3a 3b 4 5 13 14a 14b ] 4c

-3.2 0.8 -0.4 -0.9 0.0 -0,7 -0.4 -2.3 -1.7 -0.9 -0.2

-22.8 -24.4 -24.2 -

0.22 1.13 0.19 0.14 0.10 0.85 0.21 0.71 0.71

41 42a 42b 43 44 46a 46b 47a 47b 48a 48b

-0.7 0.2 0.5 0.1 -0.1 -0.3 -0.3 -2.2 -0,4 -4.5

-25,8 -24.2 -24.6

0.t8 0.15 0,16 0.17 026 0.] 8 0.21 0.29 0.13 0.22 O.12

t 5a

-2.0

-25.1

0.45

48c

0,1

-

0.21

15b 16 17 18 19a 19b 27a 27b 28a 28b 29a 29b 30a

0.6 0.3 -1.0 -0.3 -0.t -1.1 -18,9 0.7 0.3 0.1 -1J 0.3

-25.7 -22.0 -

0.63 0.71 0.22 1.05 0.26 0.20 0.20 0.19 020 0.49 0.26 0.64 0.37

49a 49b 50a

0.8 1,0 0.1 -1,5 2.2 -5.1 -5.4 -5.0 -1.7 -4.0 -5,8 05 2.0

-23.4 -26.3 --24.8 -25.9 -21.9

0.I 0 0.19 0.13 0.16 0.66 0.34 0.42 0.43 0.83 0.69 0.46 0.13 0.32

0.6

50b

63 65 65a 65b 66 67 68 70 71

0.21

o r calcite) w h e r e t h e c e m e n t i t s e l f e x h i b i t s a s e c o n d a r y ( n e o f o r m e d ) s t r u c t u r e . S u c h m i n e r a l o g i c a l n e o f o r m a t i o n s are g e n e r a l l y o b s e r v e d a l o n g t e c t o n i c f r a c t u r e s w h e r e relatively l o w 613C v a l u e s are a r e m a r k a b l e f e a t u r e (Fig. 7.3). The neoformed carbonate minerals are paragenetically associated with organic m a t t e r o f a n e p i g e n e t i c n a t u r e . T h e latter is l o c a t e d e i t h e r in t h e f i s s u r e s o f t h e r o c k o r in m a s s i v e f o r m o v e r t h e fields o f r e c r y s t a l l i z e d c a r b o n a t e s . U n d e r t h e s e c o n d i t i o n s s o m e t i m e s " f r e s h " p y r i t e s is e n c o u n t e r e d . T h e i s o t o p i c c o m p o s i t i o n o f t h e c a r b o n f r o m t h e r e c r y s t a l l i z e d c a r b o n a t e s in t h e s e f i s s u r e s i n v a r i a b l y e x h i b i t s 613C v a l u e s w i t h a p r e d o m i n a n c e o f t h e l i g h t e r i s o t o p e ( < - 2 . o % o ) . T h e i s o t o p e c o m p o s i t i o n o f t h e org a n i c c a r b o n f r o m t h e s a m e s a m p l e s is 2-3%o l i g h t e r t h a n t h e o r g a n i c c a r b o n f r o m m i c r i t i c r o c k s o r f r o m c a r b o n a t e s c o l l e c t e d away f r o m t e c t o n i c f r a c t u r e s , In o u r o p i n i o n , this v e r y case, like t h a t o f t h e u p p e r z o n e s i n t e r s e c t e d b y d r i l l h o l e HM-1, clearly s h o w s t h a t t h e r e h a s b e e n a n o x i d a t i o n o f t h e o r g a n i c m a t t e r t h a t h a s m i g r a t e d in t h e f o r m o f h y d r o c a r b o n s . T h e i n t e r a c t i o n o f t h e CO 2 t h e r e b y f o r m e d w i t h t h e r o c k s r e s u l t e d in a l i g h t e r i s o t o p i c c o m p o s i t i o n o f t h e total c a r b o n a t e c a r b o n . I n g e n e r a l , it c a n b e s a i d t h a t i s o t o p i c a l l y l i g h t c a r b o n a t e s are a s s o c i a t e d w i t h r o c k s rich in e p i g e n e t i c o r g a n i c m a t t e r .

7.2 • General Data on Carbon Isotope Composition of Rocks and Organic Matter

261

o-

o

L

,=

~I

~

262 Table 7.4. Isotope composition of carbonate carbon and organic carbon from the AYn-

Touta and Djidjelregions

Chapter 7 • Degree of Preservation of Hydrocarbon Accumulation

No. of sample

6~3Ccarbonate (%0) 613Corganic (%0)

AT-1 AT-2 AT-3 AT-4 AT-5 DJ-1 DJ-2 DJ-3 DJ4 DJ-5

-0.4 -1.2 -3.0 2.4 -2.8 -0.3 -2.1 -1.6 -0.1 -5.4

-25.4 -25.1 -22.6 -25.4 -23.5 -23.8 -22.8 --25.6

7.2.1.3 The AYn-Touta (Batna) and Djidjel Areas The carbonate carbon isotope compositions of the Mn-Touta (Batna) and Djidjel areas are similar to each other (Table 7.4, Fig. 7.2), with the majority of the 6~3C in the samples being negative. They range from -o.1 to -5.4%o for Djidjel and from +2.4 to -3.o%0 for the A'/n-Touta region (Fig. 7.2). The regularities observed in the preceding regions of Djebel Hamra, S~tif and M~djounbs are encountered also here. The lowest 6~3Cvalues are notably found in recrystallized carbonates (calcite, dolomite) and especially in those associated with epigenetic organic matter. The paragenetic associations (recrystatlized carbonate + epigenetic bitumen) are located in the fissures where they sometimes occupy entire fields which still sometimes contain "islands" of not transformed micritic carbonate. The 6~3Cdata obtained in these two regions are, from a methodical point of view, of particular importance as clear indications of petroleum have been noted in the samples studied, viz. oil in a sample from Mn-Touta and gas in samples from Djidjel. This is proof of a concrete factor for the reliability of the indications furnished by the carbon isotopes for petroleum exploration in northeastern Algeria.

7.2.2 Carbon Isotope Composition of Organic Matter The carbon isotope compositions of organic matter from the five regions studied exhibit a narrow range of -21.9 to -26.3%0 (Tables 7.1, 7.z, 7.3, 7.4; Fig. 7.4), the lightest organic carbon coming from samples taken close to faults (Fig. 7.5). In those samples the organic carbon is a-3%o lighter than the organic carbon from micritic rocks or from carbonates derived from zones removed from the fault zones. There is, furthermore, the following trend: the lightest isotope composition in organic carbon was noted in samples distinguished by a high degree of recrystallization of the carbonate material which concurrently exhibit "lighter" carbonate carbon. A comparison of the 6uC of the carbonates with that of the organic carbon implies a direct link between them. The coefficient of correlation for all data appears to be fairly high, with the mean values being o.93 for Djebel Hamra, o.83 for S6tif, o.89 for M~djoun~s and o.97 for A~n-Touta and Djidjel. There is no correlation between the

7.3 . T h e M e c h a n i s m

(~13 C (O~o) -,32

-31

of Stable Carbon Isotope Fractionation

.30

~29

-28

-27

-26

-25

~63

-24

-23

-22

-21

-20

-19

-18

Isotopic composition of organic carbon Djebel Hamra . . . . . .

~ . ~ m

~

S~tif

~ r~ ~

r~r~r~__

M~cljoun~s

~

r ~

~ r ~

Ain Touta (Batna)

~

~

~

~

Djidjei

r~:

m

~..

_ _ ~ _

.....

. . . . . . . . IS 13

'13C'12C' ~ /~chant.

t

C= 61~C (%0) -32

-3~

-30

49

~ Oil

'~3C'120 / IPDI~

- t

if3CC/'~

3

I

N Carbonate carbon

--x10 2a

~ _ _

-27

-26

~ Organiccarbon -2s

-24

-2s

-22

-21

-20

-19

-18

Fig. 7.4. Variations in carbon isotope compositions

content of organic carbon and the isotopic composition either of the organic carbon or of the carbonate carbon. The data described above are further confirmation of the secondary migration of petroleum-type hydrocarbons across tectonic dislocations where oxidation of the latter led to secondary carbonates with varying degrees of "lighter" isotopic compositions ((Makhous 1979). As in the case of the carbonate carbon, the low variability of the absolute isotope values of the organic carbon (Fig. 7.4) and the low abundance of the lighter isotope over the area (Fig. 7.4, 7.5) indicate a rather limited migration of hydrocarbons through the succession studied as well as their ultimate destruction.

7.3 The Mechanism of Stable Carbon Isotope Fractionation (12C VS. 13C) and Regularities in Their Distribution in Jurassic and Cretaceous Deposits of Northeastern Algeria 7.3.1 Decomposition and Oxidation of Organic Matter As already pointed out, the available data indicate a rather narrow variation of (~I}C as well as a lower presence of 12C in the study area (Figs. 7.2, 7.6, 7.4, 7.5). This implies a mechanism of formation of 613C controlled by an equilibrium of the respective isotopes, viz. ~2C and ~3C of different origin which were encountered in the recrystallizational dnvironment of the carbonates. The carbon dioxide (C02) had to be formed through destruction of the organic matter, a process taking place during bacterial activity which produce CO2 by assimilation of organic matter and in particular of petroleum. Under aerobic conditions the

264

Chapter 7 • Degree of Preservation of Hydrocarbon Accumulation

L~

o

'/

~o

.Q

~6

o

r~ u~

o =

,¢ o

o

7.3 . The Mechanism of Stable Carbon Isotope Fractionation

a65

organic matter can be oxidized by free oxygen to furnish carbonic acid. The process of diagenesis of organic matter can take place along two distinctive pathways: 1. Decomposition: CxHyO~

> CO~ + C~_~HyO~_~ ,

z. Oxidation: C~HyOz+ O~

> C02 + Cx_lHyOz •

The intermolecular isotopic heterogeneity actually controls the differences in the formation of the isotopic composition of the carbon in the C02 resulting from the decomposition of organic matter (1) or from oxidation (2). The carbonic acid produced by the decomposition of organic matter and inheriting the carbon of the functional groups will be enriched in the heavier isotope because of the preferred concentration of ~3C in such groups. However, the carbonic acid formed by oxidation of organic matter possesses a carbon isotope composition corresponding to the mean isotope composition of the organic matter oxidized, i.e. it is characterized by the predominance of ~C over 13C. Process (I) takes place under anaerobic conditions and process (2) under aerobic conditions. It is obvious that under the specific geochemical conditions of northeastern Algeria, i.e. an oxidizing surface environment or penetration of atmospheric agents along faults, (aerobic) oxidation of hydrocarbons inevitably wilt lead to the formation of CO2 enriched with 1~C. The same carbonic gas with light carbon will, on encountering surrounding micrite carbonates, the isotope composition of which is characterized by a predominance of ~3C,produce a sort of leaching effect on the latter. We thus will have in solution bicarbonate of organic derivation together with carbonate ions from the dissolved initial carbonates: Ca~3CO3+ 1~CO~+ ttzO ------7Ca z+ + H13C03 + H~2C03 , CaMg(13CO~)2 + 21zC02 + 2H~O ..... > Ca -,++ Mg~+ + 2H13C0; + 2H12C0~ . The secondary carbonate recrystallized from these solutions may inherit to the same extent the isotope composition of the primary micrite carbonate as well as that of the oxidized organic matter. It is evident that the isotopic composition of the secondary carbonate will be controlled by the ratio of H12C03 to H13CO3in solution and thus by the degree of oxidation of the organic matter. This mechanism for the formation of a certain 613C is in perfect agreement with the results obtained in our study areas. However, the 613Cvalues observed in the lower portions of drill hole HM-1 (2 000-4 o69 m) as well as in certain isolated instances require further clarification. These essentially positive values could not have been established under participation of C02 resulting from the oxidation of organic matter as this should have led to a more or less pronounced predominance of 12C. Most of the samples collected over this interval and from certain other regions represent only non-transformed micrite carbonates. XRD analyses and microscopic observations only reveal very poorly crystallized limestones, the carbon isotope composition of which corresponds to that of carbonate rocks of marine origin. Even here,

266

Chapter 7 • D e g r e e

o f Preservation o f H y d r o c a r b o n A c c u m u l a t i o n

however, one sometimes observes fissures or fields filled with well-crystallized calcite or dolomite and associated with bitumens. The 613Cvalues of such neoformations appear to be rather elevated. This anomaly of positive values may be explained by the leaching of primary micritic rocks by CO~ derived from the decomposition of organic matter under anaerobic conditions by bacterial action. This process, as has been described above, leads to CO2 which inherits the '3C-enriched carbon of the functional groups.

7.3.2 Methanogenic Fermentation Carbon dioxide enriched with 13Cmay also form together with methane during methanogenic fermentation: CxHyOz ..... > COz + CH 4 + Cx_2gy_4Oz_ 2

Taking into account the biogenic isotope equilibrium, the COa and C H 4 resulting from the metabolisms should obtain different isotopic signatures, CO2 being enriched with I3C and CH4 with 12C. Traces of methane have actually been detected in the pores of certain samples from HM-1 with the aid of mass spectrometry of gas inclusions (Table 7.5). This CH 4 exhibits a rather light carbon isotope composition and could even have participated in the establishment of the 313C of the recrystallized carbonates in the upper parts of HM-L However, the observed traces of C H 4 are associated with greater concentrations of hydrogen (Ha) and nitrogen (N~) which would imply a primary origin of the methane (Table 7.5). The paragenetic association of CH4, H a and N~ is actually characteristic of brines of organic derivation. The joint occurrence of CHa, H~ and Na may be considered as supplementary evidence for the existence of secondary migration of hydrocarbons in the respective area.

7.3.3 Sulfate Reduction by Bacterial Activity We shall now discuss the rather interesting unique case of sample 27a which is made up of recrystallized isotopically very light carbonate. In the respective western part of

Table7.5. Composition (%) of gaseous phases occluded in rocks of drill hole HM-1

D e p t h (m) 1334.4- 1335.0

H2

N2

-

64.7

35.3

-

100.0

50

40

1429.0- 1430,3

Traces

2683.2 - 2686.4

10

3506.0- 3571.0

57

4067.0 - 4068.0

-

29 Traces

CO 2

Traces (very little gas)

CH a

14

7.4 - Conclusions

267

the M~djoun~s structure the carbonate carbon exhibits 313C of -18.9%o. Such a 8~3Cvalue corresponds to the reduction of sulfates like gypsum and anhydrite where the activity of sulfate-reducing bacteria, in the presence of hydrocarbons, forms H~S: CaSO 4 + 2H20 + CnHn+2

> CaCO3 + CO2 + Ha0 + H2S

In continuing the reaction, this hydrogen sulfide will be oxidized by oxidative infiltrating waters to result eventually in elemental (native) sulfur according to the following formula: 2H2S + O2

> 2S + 2H20

It is during this reaction corresponding to the model of sulfate reduction and oxidation of organic matter that the very"light" calcite is formed. It is clear that such a calcite wilt only inherit the isotopicatly light carbon coming from the hydrocarbons in the absence of micritic 13C-rich carbonates. Such examples of the association of isotopically very light calcite with native sulfur are also known from regions like the USA, Mexico, and the Middle East (Makhous 1974). To sum up, irrespective of the mechanisms leading to the isotopic compositions in the regions studied, the variations of (organic or mineral) 813C are very narrow. Furthermore, the abundance of the light isotopes is quite low (Tables 7.1, 7.2, 7.3, 7.4; Figs. 7.2, 7.4) and is restricted to the zones of secondary hydrocarbon migration. These observations may serve as indicators of a rather weak oxidation (destruction) of the final hydrocarbon accumulations in the province concerned. 7.4

Conclusions 1. The peculiarities of the carbon isotope composition in the study area are explained by a model of the formation of"light" carbonates by the intrusion of organic carbon consisting essentially of the light isotope ~2C.This intrusion took place after the oxidation of migrating hydrocarbons under aerobic conditions or under the influence of sulfate-reducing bacteria. Irrespective of the oxidative mechanism, the carbonic acid formed inherited the light isotope from the carbon of organic derivation. The final carbon isotope composition of the recrystaltized carbonates (calcite, dolomite) is controlled by the ratio in the solution of the H12CO3- ions derived from the oxidation of hydrocarbons to the H ~ 3 C O 3 - ions derived from the leaching of primary micritic carbonates enriched with 13C.Consequently, the carbon isotope composition (8~3C) and its variations may be used as a direct indicator of degree and even the extent of the oxidation or rather destruction of the hydrocarbon accumulations. 2. The fiuC values observed for the different regions indicate a lower abundance of the light isotope 12C on surface as well as at depth. Furthermore, the variations of the absolute 813Cvalues are also rather limited, the latter ranging from -7.5 to +4.5%o for the carbonate carbon and from -27 to -26.3%o for organic carbon. These data, on the whole, show that there is only a small volume of"light" carbonates which actually is in agreement with the insignificant amounts of petroleum destroyed

z68

3-

4.

5.

6.

7.

Chapter 7 . Degree of Preservation of Hydrocarbon Accumulation

during its secondary migration from accumulations (or deposits) located in deeper underlying strata. In other words, the real hydrocarbon accumulations should have been well preserved. This conclusion may be considered as valid and may serve as an argument in favour of exploration for hydrocarbons in northeastern Algeria. A great similarity in the carbon isotope composition of 613C in samples collected in the different areas of northeastern Algeria has been noted. Taking into account this observation and the similarity of the geological situations in these areas, we may assume a uniformity of the geochemical processes which took place there and of the secondary migration, and, consequently, the existence of one large petroliferous province. In view of the great uniformity of the operation of the processes of migration and oxidation of organic fluids, relatively negligible variations in the carbon isotope composition should be expected. Any such variations are controlled by a variety of factors such as local geological and tectonic peculiarities, different quantities of organic fluids available, the mechanisms of oxidation, pH of the environment, etc. A regular trend for a heavier carbon isotope composition (organic and mineral) has been observed with increasing distance from organic accumulations and zones of hydrocarbon migration. This regularity underlines the fact that the presence of "light" carbonates due to oxidation of hydrocarbons may be considered as evidence, in addition to the geochemical and mineralogical criteria established previously by Makhous (1982),of the confirmation of zones of secondary migration. The greatest concentrations of recrystallized carbonates with the light isotope 12C directly indicate the zones of secondary migration themselves. The probablity of finding such carbonates with the light isotope 12C is much higher than observing traces of hydrocarbons or other paragenetic formations, considering the higher stability of the recrystallized carbonates. From the data obtained we may recommend the use of a 6~3Cvalue of -2%o or still lower in the carbonate carbon as an indicator of accumulations (deposits) of hydrocarbons in the carbonate rocks of northern Algeria. The exceptionally "light" carbonates (6~3C= 18.9%o) in the M6djounbs structure result from sulfate reduction by sulfate-reducing bacteria in the presence of hydrocarbons, leading eventually to the formation of elemental (native) sulfur. In view of the presence of thick layers of gypsum (CaSO4 x 2H20) and anhydrite (CaSO4) in the succession at the above-mentioned site (sample 27a), a potential for the presence of a native sulfur deposit is indicated. The practical importance of carbon isotope analyses lies in the possibility to distinguish carbonates formed (or transformed) during migration and oxidation of hydrocarbons from sedimentary or other carbonates even in the absence of real mineralogical differences.

Chapter8

Reconstruction of Temperatures from Organic and Mineral Diagenetic Criteria

8.1

Reconstruction of Temperatures from Degree of Structural Ordering in Mixed-Layer Minerals The mixed-layer minerals of the illite-montmorillonite type (I/M) in the shales as well as in the sandstones are characterized by an increase of the illite content with depth. It has been noted that the I/M mixed-layer minerals in shallow-buried sandstones contain much less illitic layers than the I/M coming from the shales. This difference in the illite content between sandstones and shales is controlled by the fact that the I/M in the sandstones are largely of authigenic origin and formed as cement in equilibrium with the physico-chemical conditions prevailing in the porous rock during their precipitation (Clauer et al. 1992,1994). As a consequence, the variation in the primary composition of the I/M is rather limited. It may be modified later during diagenesis when the environmental conditions change. In the shales the I/M are of detrital derivation and thus possess a varied composition. During burial and the concomitant increase of temperature the compositions of I/M in shales and sandstones become increasingly similar to each other. Ktibler (1993) has pointed out that the transformation of the swelling layers in mixed-layer clay minerals is controlled by reaction kinetics and permeability and thus an increase in temperature will speed up the reaction. An important role in the illitization of smectite is attributed to the compaction which reduces the permeability of the argillaceous and silty-argillaceous intercalations (Kiibler 1984). The proportion of the illitic layers in the I/M increases with depth and the unordered phases are transformed into ordered phases of the allevardite type (Fig. 8.1). This transition is easily recognized in diffractograms of samples saturated with ethylene-glycol by the disappearance of the 17-A peak and the appearance of a peak at 13-14 A indicative of a structural ordering in short chains (Fig. 8.2). Such a phase transformation has been initiated at various levels in the Oued el-Mya Basin at depths of 2 3oo-2 5oo m. The transformation of I/M of the allevardite type into those of the kalkbergite type with >8o% illitic layers in the lattice takes place at a depth of 4.3-4.8 km in the Paleozoic sediments. Corrensite, a mixed-layer mineral of the chlorite-montmorillonite type with an ordered structure, occurs at several levels within the Triassic Basin and in particular in the area of the Hassi R'Mel deposit (Plate 15). Corrensite is a highly useful geothermal indicator in sediments (Porrenga 1967; Ktibler 1973). In the area mentioned it starts to appear at a depth of 2.1 km and remains stable down to 2.3 km. The maximum temperatures reached were reconstructed on the basis of the appearance or disappearance of allevardite, kalkbergite and corrensite mixed-layer minerals (Fig. 8.2). Mineralogical and crystallochemical analyses of mixed-layer clay minerals reveal the pro-

270

Chapter 8 - Reconstruction o f Temperatures

Unordered phase

~ Well-ordered crystall structure

,..

.q.'.'.-'..'"

-oo-

C

~

-

x-

_x_.-x-,~ ~,,c~.x. * -x-_K_ ~ _

•c- -c>

-~-

~-

Illite degraded, pooriy crystallised 1 1O0

80

60 40 Smectite layers (%)

20

0

20

40 60 Illite layers (%)

80

Illite agraded, moderately crystallisect i

1O0

9

Illite degraded, poody crystallised

8 7 6 5 Half-width of peak at 10 A (mm)

4

3

Fig. 8.1 a. Proportion ofillite-smectite layers in crystal structure of mixed-layerminerals ( C ~

"~I~_a_-Q-~cL~- -0- _%-0r r e~, n s i t

e

~.~,%'~C~?

-o=o .

-~3

(Short-range o r d e r &

.-~TclayJ~_ ~ -o- ~

. . . .

\

\\-I -o-

(Long-range ordering) >85% illite layers in I/M

>300°C

Pyrophyllite 20 20 l

40

60

80

I00

40

60

80

100

n

r

I

Illite layers in I/M (%)

Chlorite layers in Ch/M (%)

- -

8.2

Crystallographic Features of Clay Minerals as Thermal Indicators in Petroleum Geology The kaolinite mineral species studied are kaolinite, kaolinite d (disordered kaolinite), dickite and nacrite. These polytypes have been described by Bailey (1963) on the basis of sense and degree of displacement of 1 : 1 layers and the position of vacant octahedral positions in the layer sequence. For the hydrated kaolinitic minerals, we have used the terminology of Keller and Johns (1976) which is based on endellite as the completely hydrated species and hatloysite as the partly or completely dehydrated species. The potytypes of chlorite have been described by Bailey and Brown (1962) and Hayes (197o). In Fig. 8.3b it is shown that montmorillonite, the mixed-layer clays and illite are located between pyrophyllite without interfoliar charge and the dioctahedral

272

Chapter 8

. Reconstruction of Temperatures

8.2 - Crystallographic Features of Clay Minerals as Thermal Indicators in Petroleum Geology

273

mica with a deficit of 1.o equivalent in the interfotiar charge for each 01o(0H)2radical (Lanson and Champion 1991). The montmorillonites possess a charge deficit in the range of 0.2-0.4 equivalents for each 01o(0H)2 radical in a layered structure of type 2 : 1,leading thereby to a swelling of the structure. Illite exhibits a charge of 0.8 equivalent for each 01o(0H)2 radical, a charge that is too high to provoke swelling of the structure when potassium occupies an interfoliar position. Illite thus resembles muscovite. Anyhow, two types of the mixed-layer minerals illite-montmorillonite may be distinguished in particular: (1) the allevardite (IMIMIMIM...) with an ordering of short chains usually encountered when the clays contain less than swelling layers (of montmorillonite) and (2) the kalkbergite (IMIIIMIII...) with ordering in long chains. Corrensite is characterized by an ordered alternation of the mixed layers of chlorite and montmorillonite.

Argillaceous Rocks. As a general rule, solid mineralogical criteria for the increasing degree of mineral maturity include: (1) the transformation of montmorillonite to illite via a sequence of mixed-layer minerals of the montmorillonite-iUite type, (2) the appearance of chlorite, and (3) the disappearance of potassic feldspars by decomposition. These mineral transformations may be described by the following reactions: montmorillonite + K-feldspar

) illite + chlorite + quartz

The proportion of illitic layers in the structural series illite-montmorillonite is the most sensitive indicator of the metamorphic degree shales. Furthermore, the illitemontmorillonite is characterized by a series of mineral species starting from disordered alternation, passing through ordering in short chains and then to ordering in long chains in the illite and montmorillonite layers. The transformation of illite to dioctahedral mica represents the culmination in the metamorphism of pelitic rocks prior to the green schist stage (Eber11993). In Fig. 8.3a we present the general mineral transformations observed in the Saharan basins as a function of the degree of metamorphism resulting from the burial of their argillaceous rocks.

Silty-Sandy Rocks. The diagenetic associations of clay minerals in the sandstones are more varied than in the shales which is certainiy due to the much greater permeability of the sandstones in comparison with that in the shales. The chemistry of the interstitial waters in the shales is largely controlled by the composition of their solid mineral matter and in particular by the decomposition of unstable detrital material, by the (potential) existence of solutions which are in equilibrium with the stable phases and with filtration processes through a membrance (Berry and Hanshaw 196o). The high permeability of the sandstones leads to a regime in which the interaction of the solution with the solid phase is of importance, the phases forming here being largely determined by the composition of the solution. The clay minerals encountered in the Saharan reservoir rocks and formed during diagenesis from the solutions include

Plate 15. Claymineralsof the chloritegroup,a, b, g, h Authigenicchlorite;c, d corrensite;e,f chamosite

274

Chapter 8 • Reconstruction

Fig. 8 . 3 . a Correlation of the temperature-dependant clay mineral assemblages in shales and sandstones, Saharan basins; b Distribution of montmorillonites, illites and mixed layers (I/M) within the compositional triangle pyrophytlite muscovite - celadonite (glauconite)

of Temperatures

i-- Montmorillonite .............

+

I-- (I/M) Random ~--+ ( I / M ) A l l e v a r d i t e

"~

--,--.--~ --. ( I / M ) K a l k b e r g .-- i

~--.lllite (IM)

2M Mica

Pyrophyllite Chlorite

t~ .....

~

..... ........ +. . . . . .

............

--

.... ~

Kaoline

K-Feldspar

~- - - 1 b d C h l o r i t e

......

~

~ ...... lib Chlorite

..... ................

Halloysite/kaolin

~ ......

~. . . . . . ,

t 0

d

Kaolinite r.....

-- + --- ~

lb Chlorite

......

+. . . . E. . . . . . .

m

Chamosite Dickite/nacrite Corrensite

IM lllite

a 0

,

T

50

100

T - -

150

+

i

-r-

200

250

300

Temperature

b

350

(°C)

Muscovite

3 Celadonite (glauconite)

Pyrophyllite

chlorite lb, ta dickite/nacrite and IM-illite (Fig. 8.3, 8.4; Table 8.1). The clay minerals identified as products of the metastabte decomposition of older minerals include kaolinite (from feldspars), corrensite (from ferromagnesian minerals), altevardite, kalkbergite and illite (from montmorillonite). From studies of Paleozoic sediments of the Russian Platform, Shutov et al. (197o) developed an hypothesis of the progressmg transformation of mineral phases of the kaolinite type with increasing subsidence. They concluded that the kaolinite mineral in little-compacted sediments is kaolinite d.

8,2 - C r y s t a l l o g r a p h i c

Features o f Clay M i n e r a l s as T h e r m a l I n d i c a t o r s in P e t r o l e u m G e o l o g y

~

c

°°.~:~ ~ ~

.~_~

~.~-~

.~_~7,:~

o

~E

u o

275

u Ill o ~ m c

~

8= ~_

o

o~

--

~

~

×~= -~.~

~.7

-~

E

g.

~

o

-~ ~ o ~

v ~.~

~_~.

k~ o

E

~

~ "E ~ .~

6 °

° u

~,

o

o

_

~.~

~

o

~-~

°

_i

',P

o,

I~

n

,., ,=,~

I

i i

>.

--

~

o

E % O~ ~,~ 0

~-~

+~ ~ - o ~ ' o v

~01

~'~_ O ~

,7+~7 " ~ '--- ~

-~ - 'o

o ~

. . . .

~Oo

,~

\.1

I

~,~

~'_--_--~¢ \ ,

-

.

b

"--

~

\

.~7 1

.....

\

/

/

,.,.=

~- u O ~" -

+~o

~ "" ;_~

,.~_~ :'~'0 "~

=

-o

.~

I ,I

"~~/

l~i-=~\i

t

t

0 u "~k...,A..~ v~

~-I"-1

o

I

-~oo

, okaolinite d ( in slightly compacted sediments) >kaolinite (in shallow-buried sediments) >dickite/nacrite (in deeply buried sediments).

In this sequence the authors ignored halloysite/endellite, important to dominant kaolinite minerals resulting from the alteration of feldspars in recent sediments. We propose the following diagenetic sequence: halloysite/kaolinite d

> kaolinite

> dickite/nacrite.

For the minerals of the chlorite group we arrive at the following sequence: 1. The species prior to the green schist stage are all chlorites of type I. z. Chlorite of type Ibd (Ib with disordered structure) may not be stabte above 7o-8o °C. 3- The transformation of type Ib into IIb catagenesis is contemporaneous with the lithological transformation of the pelites, i.e. it takes place in the sequence: shales >slates and illite >dioctahedral mica of type 2M (zoo °C) (Figs. 8.3, 8.4).

8.3 - Summary

277

Corrensite is present in several genetic types as described by Kiibler (1973). It has been observed in the Triassic dolomitic limestones associated with evaporites and in volcanic sandstones which experienced some burial. Correlations with other data like Ro or vitrinite reflectance and the temperatures measured in the holes show that the transformation of volcanic ferromagnesian minerals into corrensite takes place at a temperature of 9o-loo °C at which the haphazardly arranged layers of montmorillonite in the chlorite-montmorillonite structure largely disappear. Corrensite is everywhere encountered in complexes, the temperatures of which, as measured in the boreholes, went up to 148 °C. The results of our studies of the transformation of clay minerals and of the correlation between temperature and the different diagenetic minerals have been compiled as a model in Fig. 8.4. We want to stress that the differences in the nature of the monotonous mineral complexes in the shales and of the mineral associations in the sandstones are controlled to a great extent by the composition of the interstitial waters and, in the case of corrensite, they are indicators for detrital ferromagnesian minerals. Furthermore, the absence of a certain mineral does not imply that the respective sandstone has not been subjected to the corresponding diagenetic stage during which this mineral would have formed. In the sandstones it is the presence of such an indicator mineral that is of interest. 8.3

Summary The clay mineral associations may be used as indicators of temperature and degree of metamorphism reached by their enclosing rocks (Ktibler 1964, 1973). The chemical composition of the rock and of the interstitial fluid together with the mineralogy of the terrigenous rocks also furnishes characteristic indicators for the determination of the mineral associations formed at these temperatures. The association of the various clay minerals is also a function of time and of the duration of the reactions in the case of lower temperatures (Clauer et al. 1994). The mineral species most useful as geothermometers are: 1. in shales: illite-montmorillonite, allevardite, kalkbergite, illite, pyrophyllite and chlorite; 2. in sandstones: chlorite (chamosite), chlorite-montmorillonite (corrensite), dickite and illite; 3. in volcanic rocks: chlorite-montmorillonite and zeolites. In pelitic sediments the transformation of montmorillonite into illite and its subsequent recrystallizition into type IM-muscovite facilitates the subdivision of the respective rocks into zones of different temperature ranges. The mineralogy of the shales, i.e. appearance of certain structural polytypes, crystalline peculiarities, or ordering in mixed-layer clays, as well as vitrinite reflectance Ro in several parts of the Oued el-Mya basin (Allal and Hassi Messaoud-Agreb domes) and even in the Ghadames Basin confirm that the paleotemperatures were higher than the present ones which were determined in boreholes in Paleozoic sediments of the domes mentioned.

Chapter 9

General Conclusions

1. The porosity of many of the large hydrocarbon reservoirs is generally of a secondary nature, a fact that has become known only fairly recently. The favourable situation in this context is the formation of this secondary porosity prior to the hydrocarbon migration. It may be reduced considerably, but at greater depths it will be much better preserved than the primary porosity. 2. Taking into account solubility and stability boundaries of organic acids and of carbonic acid we may conclude that the organic acids are more efficient on a local scale for the dissolution of carbonate and silica cements. Where the source of the latter, i.e. the carbonic acid, is deep and further removed they may not increase the porosity by solution and removal of cement. The action of carbonic acid formed by decarboxylation on carbonate cement leads to the enrichment, in the dissolved carbonate, of the light carbon isotope inherited from the organic matter whereas the reaction of organic acids with the carbonate cement takes place under dissociation with the formation of secondary carbonate enriched with the heavy carbon isotope inherited from carbonates of mineral origin. 3. As a result of the activity of these acids enormous quantities of carbonates are transported upwards from diagenetically mature sandstones to become deposited in immature to semi-mature sandstones. Taking into account continued burial of terrigenous deposits, the carbonate cement is subjected to cyclic transformation and to upward transport, leading to the cementation of immature sandstones (at shallower depths) by carbonates and to their blockage. The primary migration of hydrocarbons generally follows on the formation of secondary porosity as during maturation of the organic matter the main phase of hydrocarbon generation takes place after the culmination of decarboxylation. This close association of hydrocarbon sources and reservoirs in time and space favours the accumulation of hydrocarbons in reservoir rocks with secondary porosity. 4. A balance calculation of secondary silica (determined with the aid of cathodoluminescence) indicates that in a number of sandstones more quartz is mobilized by pressure solution than exists actually in the form of silica cement. Intergranular pressure solution thus represents an important agent for the transfer of quartz in the sandstones. Some of the Saharan sandstones studied behave as silica "importers" during early stages of diagenesis and as "exporters" during the later stages. 5. Petrographic observations indicate that the largest portion of the silica cement could have been deposited prior to the main phase of pressure solution. In these cases the presence of early cements inhibited intergranular pressure solution and facilitated the preservation of a relatively large volume of the porosity. Intergranular

280

6.

7.

8.

9.

Chapter 9 - General Conclusions

porosity of sandstones refers to the part of the primary intergranular volume destroyed by the process of (mechanical and chemical) compaction as well as to the part of the intergranular volume remaining filled by the cement. A new mechanism for sandstone diagenesis is proposed here, a concept that implies that the transformation of silica in the sandstones takes place in its majority in the solid phase. It has been shown that dense monolithic sandstones as well as quartzites may form through development of low-energy crystal faces on the quartz grains and by twinning of crystals of the lapan- or Dauphin6-Brazilian type. This process is accompanied by the gradual reduction of porosity and permeability up to total fusion of the grains into a dense quartzitic rock. Aggregation of grains takes place by diffusion, sliding along screwn dislocations and face-to-face movement of large angles boundaries. This type of transformation of the rock in the solid phase is initiated by the reduction of the surface energy and in an open geological system the process of self-organization described above represents a comprehensive alternative mechanism to pressure solution. We were able to distinguish three phases of decompaction of an undulating nature overlapping in time and space: the first one corresponds to leaching mainly of the carbonate cement in the interval between 1.5 and 3.8 km depth and at 5o-15o °C, the second one entails leaching of carbonate, silica and alumosilicate cements between 2 and 3.5 km depth at temperatures of 8o-12o °C, and eventually the third one with intergranular pressure solution followed by removal of silica in an alkaline environment between 2.5 and 4.8 km depth and lOO-17o °C. On a geological scale these undulating phases of decompaction overlap each other. The main factors controlling the processes of compaction-decompaction of the reservoirs in the Saharan basins are: (a) development of secondary porosity, (b) establishment of an abnormal elevated formation pressure associated with the appearance of fractures, (c) formation of growth rims on quartz prior to the Mesozoic subsidence consolidating the matrix of the sandstones and making them resistant against ultimate compaction, (d) presence of Mesozoic evaporites leading to weak heating of the deposits below salts as a result of the low thermo-insulating properties of the salts and the low gravitational pressure exerted by them because of their low density, (e) early migration of the hydrocarbons which slows down and even stops the compaction of the reservoirs, (f) temperature and pressure, (g) thickness of the sandstones, (h) transformation of structural and textural features of the cement, and in particular of the argillaceous cement, and (i) tectonic processes and fracturing. The isotopic "portraits" (of carbon) observed in oil and bituminoid fractions extracted from Silurian and Devonian source rocks in the Triassic province are of special importance for correlations between oils and source rocks. Two different shapes of isotope curves have been observed in asphaltenes with light isotopes which, as shown before, are characteristic of marine source rocks. All groups of oils studied may be subdivided isotopically into two main groups which differ clearly in the isotopic composition of the five fractions of different polarity. The first group of oils closely reproduces the peak-shaped isotope curves characteristic of bituminoid fractions of Silurian age in which we also note a light isotopic composition of the asphaltenes and a narrow spread of ~'3C values. The isotopic"portraits" of the bituminoids from Devonian shales are notably different from those of the

GenerM Conclusions

281

Silurian source rocks and correspond to the isotopic "portraits" of the second group of oils. This is an important criterion in the search for oil. lo.The geochemical data on the argillaceous source rocks in organic matter of the Silurian and Devonian formations of the Triassic Basin and the Ghadames, Oued el-Mya, Illizi, Sbaa Basins, etc. reveal good correlations with the oils trapped in them. The correlation of the carbon isotope composition of the different oil fractions with the same fractions in bituminoids of potential source rock candidates, the distribution of saturated hydrocarbons (Cio), the hydrocarbon composition of gasoline (C4-C7) and the geological distribution of effective source rocks indicate that the oils from the northern and central parts of the Triassic Basin and from the south and west of the Illizi and Ghadames Basins essentially came from a Devonian source. The oils of the fields of Hassi R'Mel, Makouda, Air Kheir, Oued-Noumer, Diorf, Guellala, Berkaoui, Ben Khala and Hassi Messaoud originated essentially in the Silurian graptolite shales of the central region of the Triassic Basin. Migration probably took place along the Hercynian unconformity, filling Triassic and CambroOrdovician reservoirs on the way. 11. The majority of the oils in the southern Paleozoic province were formed during the Paleozoic when burial of the source rocks exceeded 2-3 km. However, the Paleozoic traps were destroyed by the Hercynian erosion and the hydrocarbons then were able to dissipate. During the Mesozoic, throughout the subsidence of the northern and northeastern parts of the platform, the burial depths again reached the conditions necessary for the generation of hydrocarbons. During the Cretaceous (postAptian) the depth of burial of the source rocks reached values of about 3 km which favoured mainly the formation of gas. 12. In the northern Mesozoic province the hydrocarbons were generated mostly during the Mesozoic. There are, however, numerous hydrocarbon accumulations of a Paleozoic age in the southwest of the eastern province. The most likely source rocks are of Silurian age although Devonian shales may also be considered as potential sources. The Middle to Upper Devonian shales representing source rocks are best developed in the Eastern Erg (Ghadames). However, the process of oil generation was interrupted by a reduction of the burial depth of the source rocks during the Hercynian orogeny and the subsequent erosion of the Paleozoic formations. Although hydrocarbons could have formed during the Paleozoic, very few favourable structures or traps existed at the time. In the eastern part of the Saharan Platform a large part of the hydrocarbons that formed during the Paleozoic disappeared as a result of the Hercynian erosion. 13. Crystalline features of the clay minerals can probably be used as geothermal indicators. The correlation of the temperatures calculated from the reaction of I/M and the appearance of chlorite IIb in the Paleozoic sediments of the southern Oued elMya Basin with the (uncorrected) values measured in the boreholes revealed that the enclosing sediments in the past had experienced temperatures some 3o-4o °C higher than those obtained from diagraphic data. I4. Modeling of the burial of the basin and of the formation of the hydrocarbons allows us to calculate changes in the thickness of the sedimentary successions and the thermal regime of the sedimentary cover of the platform and to evaluate the generation of hydrocarbons in the source rocks of the basin. Alternative methods for calculating the variations of the tectonic subsidence amplitude of the top of the

282

Chapter 9 - General Conclusions

basement within the framework of a local isotopic model were used for correcting the sequence of tectono-thermal events adapted to the model. 15. The attempt to locate on the Saharan Platform and interparticular in the Triassic Province traps of the non-structural type is of particular interest for prospecting in view of the exhaustion of the "inventory" of unexplored structural traps. The stratigraphic traps are more characteristic of the Paleozoic sedimentary complex because of the presence of wedging zones and angular unconformities in the respective basins. Lithological traps are developed essentially in the sandy-argillaceous formations of the Triassic and result from the facies variations characteristic of these sediments.

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